In a diurnal artificial cryptorchidism (AC) experiment intended to provide a male contraceptive method, oligoasthenozoospermia was found as early as the fourth month, accompanied by an increase in the mean percentage of abnormally shaped spermatozoa. A detailed morphologic study concerning 19 volunteers exposed for 6 to 24 months demonstrated that the overall increase of the mean percentage of abnormally shaped spermatozoa resulted from the percentage increase of elongated, thin, and irregular heads and of bent tails. This increase is probably caused by the rise in environmental testicular temperature induced by AC. A return to baseline values was observed within 12 months after the end of the experiment.