Derived from the Greek words crypto- (meaning “hidden”) and orchid or orchis-(meaning “testicle”), cryptorchidism refers to the absence of one or both testes from the scrotum.

Clinical Signs

This is not an illness, but rather a congenital condition. There is typically no clinical signs of pain or discomfort associated with the condition.

The following may be noticed by the owner:

  • Asymmetrical scrotum containing only one testis (testicle).
  • A small hard lump felt slightly further up in the abdomen.
  • The complete absence of the other testis (testicle).
  • Enlargement of scrotum on unaffected side due to compensatory hypertrophy.


Cryptorchidism is a condition where the testes (testicles) do not descend into the scrotum but remain in the abdomen or the inguinal canal. The condition is congenital, occurring during fetal development, and may be caused from any anomaly that prevents or interrupts the testes from descending normally.

Normal gonadal development occurs at about embryonic days 10 to 12 in the rat as primordial germ cells migrate from the endoderm of the yolk sac through the dorsal mesentery to the coelomic epithelium of the gonadal ridge. The gonadal transformation into testes occurs once the the germ cells have associated with the sex cords within the gonadal ridge at about embryonic day 14. At this time fetal testes protrude into the peritoneal cavity (space within the abdomen) and appear as distinct organs.

Testicular migration, the process by which the testes descend into the scrotum, is a complex process in the rat. It is thought that the testes in rats descend passively as a result of the growth of the trunk and pelvis, but little or no growth of the connection of the testis to the scrotum (genitoinguinal ligament or gubernaculums).1

In rats, the testes normally lose their position in the abdomen and descend into the scrotum at approximately 15 days 1,2 following birth. Testes that have not or do not descend may be retained in the abdominal cavity or the inguinal canal. It is important to note that throughout life male rats normally have open inguinal canals with a functional cremaster muscle that allows the testes (testicles) to migrate easily in and out of the abdominal cavity for both protection and temperature control.

Experimental fertility studies done with young male rats have shown no impaired fertility in in the presence of a developed undescended testis; but may be reduced in older male rats. However, spontaneously occurring undescended testes have the potential to be both underdeveloped and non-functional. In addition, undescended testes pose an increased risk of developing testicular tumors as well as testicular torsion and infarction and inguinal hernias. For these reasons castration (neutering) is recommended for cryptorchid rats.

*Note: Pet owners are advised not to place the male back in with a female cage-mate immediately after surgery due to the presence of viable sperm. References citing the timeline for post operative viable sperm, in rats, vary greatly from 8 days to 8 weeks post-op. Anecdotally, waiting 2 to 4 weeks post-op before placing a neutered male with an intact female has not resulted in any known pregnancies. However, a time frame of 6 weeks post neuter prior to placing a male rat with an intact female rat is most commonly practiced by pet rat owners.

It is important for pet owners to take this information into account when choosing to place the male back in with a female cage-mate, thus preventing unplanned litters.


Case History and Photos of Cryptorchidism

  • Fig 1: Unilateral cryptorchidism: 6-week-old male rat


Obtain history from pet owner.

Palpate for presence or absence of testes in scrotum.

Palpate abdomen for presence of undescended testis (testicle).



The primary management of cryptorchidism in rats is the surgical procedure called orchiectomy (also referred to as castration or neuter).

Note the following when surgery is considered:

Preanaesthetic fasting of rats is not necessary since vomiting does not occur in this species. Free access to both food and water should be provided until just prior to anesthesia (Flecknell, 1991).

Rats do experience pain with surgical procedures. The type of pain medication used post-op should be determined based on extent of procedure and the anticipated severity of pain.

  • For severe pain or first 24 hours post-op: Buprenex (buprenorphine), or Torbugesic (butorphanol).

  • For mild to moderate pain : Banamine (flunixin meglumine), Metacam (meloxicam), or carprofen. Do not use if a corticosteroid has already been prescribed.

Consideration should be given to including a broad spectrum antibiotic post op, as prophylaxis, when performing an orchiectomy.

For information regarding medications refer to the Rat Medication Guide.

Nursing Care

Provide the following post-op care:

  • A hospital cage (smaller individual cage) that includes clean non-litter bedding such as felt, soft t-shirt type material, or ink-free paper towels. Clean cage and bedding daily. Avoid litter-type bedding post-op, until healed, to prevent the chance of wound contamination or infection.
  • Provide additional warmth to maintain body temperature within normal limits. It is essential that the rat does not become overheated or dehydrated. The rat should also be able to move away from the heat source if it becomes uncomfortable. If the rat is unconscious or immobile extreme care must be taken to keep the heat low and stable.
    • You can use an isothermic product that is heated in the microwave such as SnuggleSafe┬«. Make sure to follow the product directions carefully and wrap in a towel before placing in the cage. SnuggleSafe┬« will provide heat for 12 hours before needing to be reheated. Other similar types of product may vary in re-heat time. Check directions for individual product.
    • If using a heating pad (good for long term use) use only the low heat setting, put a thick towel in between the pad and the cage bottom, and place beneath a corner of the cage.
    • If none of these options are available you can use a plastic bottle filled with hot water, and wrapped in a towel, in the corner of the cage.
  • Medicate for post-op pain as needed.
  • Give antibiotic if or as prescribed.
  • Encourage fluid intake while recuperating, such as water, Jell-O water, or electrolyte replacement drinks such as Pedialyte or Gatorade (which can be found in local grocery stores).
    • Please note that Pedialyte is only good refrigerated for 24 hours after opened, but can be frozen as ice cubes and thawed as needed.
    • *Note: a juicy type of fruit also provides an additional fluid source in the diet.
  • Contact the veterinarian if any of the following occurs: swelling, redness, or pain at the incision site, or if there are signs of increased weight loss, lethargy, or changes in behaviors.


  • Comfort
  • Increased quality of life


  • Because cryptorchidism is believed to be an inherited, recessive polygenetic trait, breeders should thoughtfully consider whether or not to continue breeding carrier parents of the defect or their offspring. It should be noted that littermates born of carrier parents may be normal, carriers, or cryptorchid males.

  1. Russel, L. (1992). Normal Development of the Testis. In U. Mohr, D. Dungworth, C. Capen,(Eds.), Pathobiology of the Aging Rat, Volume 1 (pp. 395-403). Washington D.C.: Intl Life Sciences Inst Press.
  2. Maeda, K., Ohkura, S., & Tsukamura, H. (2000). Physiology of Reproduction: Puberty. In G. Krinke,(Ed.), The Laboratory Rat (Handbook of Experimental Animals) (pp. 145-176). Toronto: Academic Press.
  3. Suckow, M., Weisbroth, S., & Franklin, C. (2005). The Laboratory Rat, Second Edition (American College of Laboratory Animal Medicine). Toronto: Academic Press.
  4. Agarwala, S., & Mitra, K. (1996). Fertility and unilateral undescended testis in the rat model. Pediatric Surgery International, 11(4), 266-268.
  5. Keeble, E., & Meredith, A. (2009). BSAVA Manual of Rodents and Ferrets (BSAVA Manuals Series). Gloucester: Bsava.
  6. Pholpramool, C., White, R., & Setchell, B. (1982). Influence of androgens on inositol secretion and sperm transport in the epididymis of rats. J Reprod Fertil, 66(2), 547-53.
  7. Additional resources used for this article listed in References of the Health section of the Rat Guide,


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