The prolactin responses to the serotonin (5-HT) releasing agent d-fenfluramine (30 mg orally) were studied in 11 male normal volunteers after administration of hydrocortisone (20 mg orally, twice daily for 10 days) using a double-blind, placebo-controlled, cross-over design. While hydrocortisone treatment significantly elevated 24-h urinary cortisol excretion, it did not lower the prolactin response to d-fenfluramine. Plasma levels of d-fenfluramine and d-norfenfluramine were not altered by hydrocortisone treatment. These findings show that following 10 days administration of hydrocortisone, the prolactin responses to d-fenfluramine are not changed.