The effects ofsubstance P,cholecystokininandneuropeptide Ywere examined onrabbitdistalcolonicmotility. All three agents produced increased contractile activity but the mechanisms responsible differed depending on the agent tested. In the intact animal, peptide effects were measured under basal conditions and following exposure to atropine, tetrodotoxin and the alpha-adrenergic antagonist phentolamine. Administration of all three peptides resulted in a stimulation ofcolonicmotility. Phentolamine did not significantly effectsubstance P-,cholecystokinin- orneuropeptide Y-induced activity. By contrast, the in vivo activity induced bycholecystokininandneuropeptide Y, but notsubstance P, was nearly eliminated by tetrodotoxin. Only theneuropeptide Yresponse was partially atropine sensitive. In isolatedcolonicstrips,cholecystokinin-induced activity, but not that produced byneuropeptide Yorsubstance P, was blocked by tetrodotoxin. Atropine did not significantly inhibit any of the hormone-induced contractions.