Capsaicin evokes intestinal vasodilatation when given by close arterial injection probably by acting on primary sensory neurons. Several peptides known to occur in primary afferents also have vasodilator effects. We have used immunoblockade to test the hypothesis that the vasodilator effect of capsaicin was mediated by release of these peptides. Antisera to substance P, cholecystokinin, vasoactive intestinal peptide and somatostatin inhibited specifically and dose dependently the effect of each of these peptides given alone. Graded doses of the antisera to substance P, cholecystokinin and vasoactive intestinal peptide also produced a dose dependent inhibition of the vasodilator response to capsaicin. In contrast, administration of somatostatin antiserum enhanced the vasodilator action of capsaicin. Prior administration of antibodies to substance P, cholecystokinin and vasoactive intestinal peptide produced an 80% inhibition of the response to capsaicin. In the presence of these antibodies, and of atropine, the response to capsaicin was reduced by more than 90%. The results suggest that capsaicin increases mesenteric blood flow due to release of substance P, cholecystokinin and vasoactive intestinal peptide. The precise cellular origins of these peptides is unknown, but they may well be released from the peripheral endings of primary afferent neurons.