Cryptorchidism is a condition where one or both of the testes do not descend into the scrotum in a male animal. In these pets, the testes remain in the abdomen or in the inguinal canal (between the abdomen and the scrotum). Unilateral cryptorchidism, one testicle un-descended, is more common than the bilateral form. A heritable trait, this condition is more common in purebred or inbred dogs than in mixed breeds. It is assumed that cryptorchidism is heritable in cats. Retained testes are sterile (cannot produce sperm) and have a greater tendency to develop neoplasia (cancer). Cryptorchid pets therefore should not be used for breeding and should be neutered. Normally, the testes are in the scrotum at birth. They are small and soft, and can be withdrawn into the inguinal canal especially if the pet is frightened. The inguinal rings close around 4 to 6 months of age, which prevent the testes from descending if they have not already. There is no harm in waiting until 6 months to neuter the pet. One function of the testicle is to produce sperm. Sperm cannot develop at normal core body temperature, and this is why the testes are externalized in the scrotum. Retained testes are sterile and underdeveloped. The descended testicle in the unilateral cryptorchid pet is normal and produces sperm. A bilateral cryptorchid pet cannot usually reproduce, however this should not be assumed if an intact female is present. Retained testes are able to produce sex hormones like testosterone. Cryptorchid pets will continue to show sexual drive, marking behavior, and aggression associated with intact male dogs and cats. In the case where an animal’s cryptorchid status is unknown, such as an adult dog without a neuter surgery scar, a testosterone stimulation blood test may be performed. A resting blood-testosterone level is measured, followed by an injection of Gonadotropin Releasing Hormone (GnRH), and testosterone is rechecked 1 to 3 hours post injection. An increase in testosterone is diagnostic for cryptorchidism. In cats, penile spines are present in the cryptorchid and intact male, and their presence is a useful diagnostic tool. Retained testes are more likely to develop neoplastic changes. Sertoli cell tumors produce increased levels of estrogen and may cause feminization of the male. Other signs of hyperestrogenism are symmetrical hair-loss along the back, increase in breast tissue with or without lactation, bone marrow dysfunction, and changes in the prostate gland. Metastasis (spread of cancer to other organs) occurs in about 10 to 40% of Sertoli cell tumors. Castration is curative when performed early in the progression of Sertoli cell tumor formation. Bone marrow suppression (anemia, decreased white blood cell and platelet count) is often irreversible when the effects of the tumor are severe.