The possibility of direct inhibition of testicular endocrine function by carbamazepine, phenytoin, or valproate was tested using the rat Leydig cell model in vitro in order to find whether such an effect could contribute to the reduction of plasma free testosterone levels reported in male patients who take antiepileptic medications. With maximally stimulating concentrations of human choriogonadotropin (hCG) or cyclic AMP, testosterone formation was inhibited by 50% in the presence of 40 microM carbamazepine, 350 microM phenytoin, or greater than 1 mM valproate. With submaximally stimulating concentrations of hCG, leading to physiological testosterone secretion rates, half-maximal inhibition occurred in the presence of 15 microM carbamazepine, 180 microM phenytoin, or 900 microM valproate. Only the values for carbamazepine were in the clinically therapeutic range. Further investigation revealed that carbamazepine acted primarily at a target between cyclic AMP formation and cholesterol conversion to androgens. In contrast, phenytoin acted by competitive interaction at the cytochrome P450XVII which converts progesterone to androgens. Valproate had by far the lowest potential to generate adverse effects in this endocrine system.