Male rats were injected with 5, 10, and 20 mg diphenylhydantoin daily for two months and then caged for five days with cyclic females. Thereafter the males underwent necropsy, the organs of the genital tract were examined, and found to be unaffected. Blood testosterone and Leydig cell counts were normal. A significant reduction of fertility was observed which appears to be loss of libido on behavioural grounds. Except for similar behavioural changes described in human epileptics no further reports are available concerning anticonvulsants and fertility. Our findings indicate that there is no reason to suspect that diphenylhydantoin has endocrine-mediated effects on male fertility in the rat.