Prolactin response to buspirone challenge was examined in eight normal healthy male and six healthy female volunteers. A significant increase in prolactin levels was noted around 90 min following oral administration of buspirone hydrochloride. When subjects were tested on two further occasions it was found that responses in males were consistently reproducible while responses in females varied throughout the menstrual cycle with maximum responses occurring premenstrually. It is postulated that increased sensitivity of the 5HT receptor premenstrually may explain the variability in females.