Adult male rats were made bilaterally cryptorchid for 3, 7,10, and 14 days, and the peripheral serum, testicular vein serum, and interstitial fluid levels of inhibin were measured by RIA, and compared with values obtained in intact animals. The levels of inhibin in peripheral serum, testicular vein, and interstitital fluid were significantly elevated (P < 0.01) after 3 days of cryptorchidism but declined significantly thereafter. The secretion of inhibin was also studied in vitro using isolated segments of seminiferous tubules from scrotal and abdominal testis in adult rats made unilaterally cryptorchid for 3, 6, and 12 days. The basal inhibin secretion by 3-day cryptorchid seminiferous tubules incubated at 37 C was significantly greater when compared with the scrotal seminiferous tubules at 32 C. If seminiferous tubules from scrotal testes were incubated at 37 C, the secretion of inhibin was greatly increased to similar levels observed by the 3-day abdominal seminiferous tubule cultures. In addition inhibin secretion was significantly higher when tubules from 5-week hypophysectomized rats were cultured at 37 C compared to 32 C. The inhibin secretion by seminiferous tubules from 6-day abdominal testes had returned to scrotal seminiferous tubule levels but then decreased below scrotal seminiferous tubule levels after 12 days of cryptorchidism. Seminiferous tubules from cryptorchid testes remain responsive to FSH stimulation (500 ng /ml) up to 12 days of cryptorchidism. FSH-stimulated inhibin production was increased at 3 and 12 days after cryptorchidism, but similar at 6 days after cryptorchidism, compared to the response of tubules obtained from scrotal testes. Furthermore, using seminiferous tubules from normal adult rats, FSH-stimulated inhibin production was increased by raising the incubation temperature from 32 C to 37 C. These in vivo and in vitro data suggest a dual stimulatory and inhibitory effect of higher temperature on inhibin production with an initial rise in basal and FSH-stimulated inhibin secretion by the cryptorchid testis which seems to be due to a direct effect of temperature on Sertoli cells. The subsequent decline in inhibin secretion could be due to the disruption of the seminiferous epithelium.