Substance P, vasoactive intestinal polypeptide, somatostatin and neuropeptide Y-like immunoreactivity was studied by immunocytochemistry in the wall of the blood vessels of the small intestine of the cat, rat and guinea-pig. Immunoreactive nerve fibres were localized by means of the peroxidase-antiperoxidase (PAP) procedure, by use of antisera raised against these peptides. These neuropeptide-containing nerve fibres were widespread in association with the blood vessels and especially with the dense network of perivascular nerves supplying arterioles. At the ultrastructural level, immunoreactive nerve fibres were found very close to the basement membrane of the capillary walls. No immunoreactive nerve fibres were found in the wall of the veins. The anatomical findings of the present study are consistent with the proposal that several neuropeptides could function as neurotransmitters or neuromodulators in the control of blood flow in the small intestine.