Immunoreactive nerve cell bodies and fibres in the intestine have been examined using three antibody preparations raised against 5-hydroxytryptamine. Cross reactivity studies indicate that the substance localized was an hydroxylated indoleamine. In the guinea-pig small intestine, nerve cell bodies were located in the myenteric plexus and varicose fibres were found in the ganglia of the myenteric and submucous plexus. The nerve cell bodies had prominent short, broad processes and a single long process. Similar nerve cells and fibres were found in the guinea-pig stomach and large intestine and areas of intestine that were examined in mice, rabbits and rats. Properties of the neurons were examined in the small intestine of the guinea-pig. The immunoreactive material was depleted by treatment with reserpine, but not by guanethidine or 6-hydroxydopamine in dose sufficient to deplete noradrenaline stores in axons in the intestine. No depletion of 5-hydroxytryptamine by the neurotoxin 5, 7-dihydroxytryptamine was observed. After depletion by reserpine, immunoreactivity of the neurons could be restored by application in vitro of 5-hydroxytryptamine, 5,7-dihydroxytryptamine or 5-hydroxytryptophan. The restoration by 5-hydroxytryptophan was prevented by the inhibitor of L-aminoacid decarboxylase, benserazide. After reserpine treatment, immunoreactivity was not restored by tryptophan. Uptake of 5, 7-dihydroxytryptamine into the nerves was antagonized by fluoxetine. The distribution of neurons with 5-hydroxytryptamine-like immunoreactivity was compared with the distribution of enteric amine-handling neurons that take up and decarboxylate L-dopa. This comparison indicated that there are two classes of aromatic amine neuron in the guinea-pig small intestine, the enteric 5-HT neurons and enteric, non-5-HT, amine handling neurons.