Intratesticular temperature needs to be maintained at 2-7°C below body temperature for normal function. Cryptorchidism disrupts spermatogenesis, leading to loss of all germ cells except spermatogonia, peritubular fibrosis and Leydig cell dysfunction. However, the effects of cryptorchidism on Sertoli cell functions, and inhibin-related protein production in particular, are poorly characterised, and this was investigated in the following study. Experimental cryptorchidism was induced by surgically translocating the testes in adult Sprague-Dawley rats into the abdomen via the inguinal canal and then ligating the inguinal canal under anaesthesia. Control rats underwent equivalent surgery, but without translocation of the testes or inguinal canal ligation. Both cryptorchid and control rats were evaluated 7 and 14 weeks later, equivalent to two rat spermatogenic cycles of 49 days duration. The testes were removed and fixed in Bouin`s fluid for histology or frozen for biochemical and molecular studies. Interstitial fluid was collected from some testes via a small incision in the tunica. Activin A was measured by an enzyme-linked immunosorbent assay and FSH, LH, testosterone, inhibin and follistatin measured by radioimmunoassays in serum and testis homogenates. Seven weeks after inducing cryptorchidism, testis weight had decreased by 50%, largely due to germ cell loss, while testicular fluid volume doubled. Intratesticular follistatin and inhibin concentrations declined significantly by 50% and 20%, respectively by 7 weeks, but activin A concentrations were not affected. Serum inhibin was reduced by 30%, and serum FSH increased 2-fold. These observations were indicative of reduced Sertoli cell function. Testicular testosterone concentrations increased by 40%, but total testicular testosterone content and serum testosterone were unchanged, and serum LH was elevated 2.5-fold, consistent with a reduction in Leydig cell function. All changes persisted at 14 weeks post-cryptorchidism. These studies indicate that both Sertoli and Leydig cell function are damaged in experimental cryptorchidism in the adult rat.