This study investigated the role of the 5-HT2/1C receptor antagonist ritanserin ond-fenfluramine (d-FF) induced changes in food intake, prolactin (PRL) secretion and oral temperature in 12 healthy male volunteers. The study was double blind and placebo controlled. Food intake was measured using an automated food dispenser.d-FF (30 mg) significantly reduced fat intake. While ritanserin (5 mg) had no effect when given alone it abolished thed-FF induced reduction in fat intake. In addition, ritanserin abolished thed-FF induced rise in PRL and oral temperature. The results suggest that 5-HT2 or 5-HT1C receptors mediate the effects ofd-fenfluramine on appetite, prolactin secretion and temperature in humans.