Unilateral cryptorchidism is frequently accompanied by infertility. Uncertainty exists as to whether the infertility is a genetic effect or is related to an autoimmune reaction to the elevated testis. The effects of unilateral cryptorchidism were evaluated in 50 mice by surgically elevating the left testicle of 21-day-old mice into the abdomen (AT). A sham operation was performed on the left testicle of 50 control mice (SHT). The temperature of the abdominal testes measured 2.5 degrees C higher than the scrotal testes. The testes were removed from both sides at 1, 2, 3, 4 and 6 weeks postoperation. After testicular weights were recorded, seminiferous tubule diameters were measured, and germinal epithelium maturity was graded histologically using a modified Johnson testicular biopsy score. Progressive abnormal changes were seen in the contralateral descended testicles of AT as compared to SHT. By 3 weeks, though testicular weight changes were similar, mean seminiferous tubule diameter was smaller (P less than .001), and the germinal epithelium was less mature (P less than .001). These changes persisted through the sixth week. By changing the physiologic environment of one testicle, we have induced alterations in the histologic appearance of the contralateral testicle during the period of normal maturation.