Although it has been recently shown that GnRH is capable of increasing plasma PRL levels in humans, the role played by the steroid hormone environment in influencingthis response has not been clarified. Fourteen intact and 14 castrated subjects with carcinoma of the prostate were studied before and after daily treatment with 1.5 mg estradiolbenzoate (E2B), im, for 9 days. PRL responsiveness was tested after GnRH was given as an iv bolus or a continuous infusion for 4 h. During a 4-h saline infusion after E2B treatment, plasma PRL levels were measured in 8 intact and 8 castrated subjects for control purposes. No significant increase in PRL levels was noted after iv bolus or infusion of GnRH in intact or castrated men. After the administration of pharmacological doses of EB for 9 days, plasma PRL levels increased significantly in all subjects after both the iv bolus and the infusion of GnRH. During saline infusion, a significant decrease in plasma PRL levels was observed in all subjects. Plasma gonadotropin levels showed the expected increase after GnRH administration. Our findings confirm that GnRH is one of the numerous substances capable of stimulating PRL release in humans and demonstrate that in men: 1) pharmacological doses of estrogen induce a PRL response to GnRH, and 2) GnRH elicits different patterns of PRL release depending on the modality of administration. Finally, the physiological role of GnRH in PRL release, if any, remains to be established. (J Clin Endocrinol Metab55: 1212, 1982)