Responses to galanin (GAL) and vasoactive intestinal peptide (VIP) and their distribution were investigated in muscularis externa from the human sigmoid colon. Immunohistochemical studies revealed GAL-like immunoreactivity (GAL-IR) and VIP-IR in nerve fibres supplying both muscle layers as well as the myenteric ganglia. Additionally GAL-IR was shown to co-exist with VIP-IR in many nerve fibres innervating human circular and longitudinal (taenial) colonic muscle layers. Circular and longitudinal muscle strips were contracted by GAL and relaxed by VIP. There was a marked difference in sensitivity between muscle layers to both peptides; circular muscle being less sensitive to GAL, longitudinal muscle less sensitive to VIP. When given simultaneously GAL attenuated responses to VIP on longitudinal muscle, thus in the same neurone there are substances having opposing actions on smooth muscle motility.