The effect of capsaicin injected into the superior mesenteric artery has been studied on the intestinal blood flow in dogs. Capsaicin evoked a marked dose-dependent increase in mesenterial blood flow in the dose range of 0.1-7 micrograms/kg. The intestinal vasodilatatory effect of capsaicin could invariably be demonstrated after pretreatment with adrenoceptor and dopamine receptor antagonists, as well as with the ganglion blocking agent hexamethonium. Pretreatment with atropine significantly reduced, but did not abolish the increase in intestinal blood flow elicited by capsaicin. Concomitant administration of somatostatin significantly inhibited both the atropine-sensitive and the atropine-resistant components of the effect of capsaicin injected into the superior mesenteric artery. Our results indirectly support the assumption that the intestinal vasodilatatory effect of capsaicin may be mediated by substance P release from capsaicin-sensitive paravascular nerve fibres associated with the blood vessels of the gastrointestinal tract. It is suggested that sensory substance P-containing nerve fibres may be involved in the regulation of the vascular reactions of the gut.