The concentration of phenytoin (diphenylhydantoin, DPH) was measured in plasma and semen of rabbits and man. In the rabbit, a single iv injection of DPH (4.64 mg) resulted in a concentration-time curve for DPH in semen parallel to the concentration-time curve for DPH in plasma (t1/2beta = 171 +/- 29 min). A semen/plasma drug concentration ratio of 0.20 was maintained for at least 8 hr, demonstrating that DPH concentrations in semen are directly proportional to DPH concentrations in plasma. In epileptic subjects maintained on oral DPH the mean drug concentration in semen was 2.31 microgram/ml while that in plasma was 13.8 microgram/ml. The mean semen-plasma DPH concentration ratio in man was 0.17; this closely approximates the observed ratio in rabbits.