A review of evidence indicates that experimentally induced changes in the activity of serotonin (5-hydroxytryptamine) systems are associated with pronounced changes in feeding behaviour. In general, treatments and procedures believed to lead to an increased availability of 5-HT in the synaptic cleft or which directly activate 5-HT receptors reduce food consumption, while procedures which either directly or indirectly decrease 5-HT receptor activation bring about the opposite effect. Interpretation of findings is hindered by methodological difficulties involved in the experimental manipulation of serotonin metabolism, by the lack of precise behavioural measures of feeding, and by the presence of large stores of serotonin outside the brain. However, available data favour the idea that serotonin systems play an inhibitory role in feeding, possibly in the mediation of satiety. This proposal has implications for further experimental investigations of the control of food intake, and for the aetiology and treatment of obesity.