To further explore the role of serotonin (5-HT) in anxiety, 28 healthy volunteers received in a double-blind study d-fenfluramine (30 mg, p.o.) or placebo, and were submitted to a simulated public speaking test (SPS), consisting of speaking in front of a video camera. The SPS induced significant increases in subjective anxiety evaluated by the visual analogue mood scale of Norris [MANCOVA, F(1.66,39.93) = 8.51, P < 0.001], as well as in systolic blood pressure [F(3,72) = 5.70, P = 0.001] and in heart rate [F(3,72) = 3.95, P = 0.012]. The drug decreased the anxiety factor [F(1,23) = 5.21, P = 0.032], without significantly affecting physical sedation, mental sedation or other feelings and attitudes. Also, the physiological measurements were not significantly changed by d-fenfluramine. Reported evidence shows that d-fenfluramine releases 5-HT from nerve endings and blocks 5-HT reuptake, indirectly stimulating postsynaptic 5-HT receptors. Therefore, the present results indicate that 5-HT inhibits the neural substrate of SPS-induced anxiety.