Preclinical studies suggest that 5-HT3 antagonists modulate dopamine-mediated responses in the limbic system and may therefore have a therapeutic role in psychiatry. We have examined the effect of ondansetron, a specific 5-HT3 antagonist, on the psychological and psychomotor changes induced by amphetamine in human volunteers. Nine healthy males took part in this double-blind placebo-controlled balanced-crossover study. Each subject received one of three treatments in a randomised manner: (a) placebo/placebo; (b) placebo/amphetamine (15 mg); (c) ondansetron (4 mg)/amphetamine (15 mg). Subjects were assessed for self-ratings of hunger, mood, energy, alertness, restlessness, irritability, and asked to rate the abnormality of their overall subjective state. In addition, systolic blood pressure, and performance on psychomotor tests were repeatedly assessed. Although amphetamine did not cause any significant changes in self-rating of mood, energy, alterness, restlessness or irritability, it induced a significant increase in self-ratings for overall subjective state, and a significant decrease in self-ratings of hunger. Amphetamine also caused an increase in systolic blood pressure and a decrease in the mean time taken to complete the psychomotor tests. Pretreatment with ondansetron attenuated the effects of amphetamine on hunger and subjective state, but not on blood pressure or psychomotor performance tests. These findings suggest that in humans 5-HT3 receptor antagonists may partially modify the subjective effects of amphetamine, and are in keeping with results from animal studies that 5-HT3 receptor antagonists might affect neurotransmission within mesolimbic brain regions. However, it was not possible to exclude a pharmacokinetic interaction to explain the effects of ondansetron.