The distribution of 5-hydroxytryptamine receptors of the 5-hydroxytryptamine3 type was examined in human brain post mortem tissue, using quantitative in vitro autoradiography. The selective and potent 5-hydroxytryptamine3 receptor antagonist [3H]ICS 205-930 [(3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester] was used as ligand. Highest levels of labelling were found in discrete nuclei of the lower brainstem. At all levels of the spinal cord the substantia gelatinosa was also densely labelled. In contrast, specific binding in the forebrain was very low and concentrated in some regions of the limbic system. The enrichment of [3H]ICS 205-930 binding sites in nuclei of the dorsal medulla and spinal cord is in good agreement with the proposed role for 5-hydroxytryptamine in sensory processing. High densities of 5-hydroxytryptamine3 binding sites in the area postrema support a central site of action for 5-hydroxytryptamine in emesis. Finally, the presence of [3H]ICS 205-930 binding sites in the limbic system provides an anatomical substrate for the behavioural effects of 5-hydroxytryptamine3 receptor antagonists.