Sympathetic mechanisms in the regulation of rectal tone and anal canal pressure have been investigated in the cat. Immunohistochemical studies demonstrated neuropeptide Y-like immunoreactivity in noradrenergic cell bodies of the inferior mesenteric ganglion, in nerve endings in the circular smooth-muscle layers of the rectum and internal anal sphincter, in the myenteric plexuses, and around blood vessels and anal glands. Peptide YY-like immunoreactivity was demonstrated exclusively in endocrine cells of the rectal mucosa. By radioimmunoassay the amount of neuropeptide Y in the inferior mesenteric ganglion was found to be 28.5 (17.8-66.6) pmol g-1, in the rectum 0.8 (0.7-1.6) pmol g-1, and in the anal canal region 0.8 (0.5-1.0) pmol g-1 of tissue. Neuropeptide Y (4-80 pmol kg-1 min-1 intravenously) and peptide YY (1-25 pmol kg-1 min-1 intravenously) increased rectal tone and anal canal pressure. The effects were not inhibited by guanethidine, phentolamine, or propranolol. Noradrenaline (100-1000 pmol kg-1 min-1 intravenously) increased rectal tone and anal canal pressure. These effects were blocked by phentolamine and propranolol. Electric stimulation of the lumbar colonic nerves and the hypogastric nerves (8 Hz) increased rectal tone and anal canal pressure. After propranolol and phentolamine the responses partly remained. On lumbar colonic nerve stimulation the remaining responses of the rectum and anal canal were 60% (40-68%) (p less than 0.05) and 20% (15-29%), respectively, whereas on hypogastric stimulation the remaining rectal and anal responses were 32% (20-42%) (p less than 0.05) and 31% (20-47%) (p less than 0.05). Further administration of guanethidine abolished the remaining responses of the rectum and anal canal. Since neuropeptide Y is present in sympathetic neurons and mimics non-adrenergic responses to nerve stimulation, this peptide is a transmitter candidate for the effects resistant to adrenergic blocking agents but sensitive to guanethidine. Peptide YY may also play a role in sphincter control, but it cannot be excluded that it activates mechanisms similar to those activated by neuropeptide Y.