The gonadotrophin response to the opiate antagonist naloxone administered per infusion has been investigated before and after 5 days of clomiphene treatment in 5 male subjects. Before clomiphene treatment, naloxone caused a significant increase in LH levels, however, after clomiphene, naloxone failed to induce the same effect. An important role for gonadal steroids on the opiatergic inhibitory tone of LH secretion in man is suggested.