Twelve normal male volunteers took part in a double-blind placebo-controlled study to measure the effects of d- and l-fenfluramine singly and in combination with d-amphetamine on plasma prolactin levels. Both isomers of fenfluramine were found to differ significantly in their effects from d-amphetamine, and from placebo, causing prolactin levels to rise. d-Fenfluramine produced the greatest change. There were minor differences between the level of activity of the two drugs when given in conjunction with d-amphetamine. d-Amphetamine produced a non-significant reduction in prolactin secretion. The neurochemical basis for the experimental findings reported is considered, with special reference to the use of d-fenfluramine as a specific serotonergic probe.