Changes incoloniccontractility were studied in anesthetized cats following the intravenous injection of severalpeptides. Increases in contractile activity were observed after the octapeptide of cholecystokinin (CCK-8), pentagastrin, substance P or neurotensin. On theotherhand, vasoactive intestinal peptide (VIP) caused an inhibition which, in some cases, was followed by an excitatory response. The responses produced by pentagastrin, substance P or neurotensin, but not byCCK-8, were partially inhibited by atropine. Following bethanechol pretreatment, the stimulation in contractile activity elicited byCCK-8, substance P, neurotensin or pentagastrin was markedly enhanced. Responses were also increased by pretreatment with eserine, hexamethonium or mecamylamine. Thispotentiationwas blocked by atropine. It is concluded that, following treatments which cause an increase in the level of cholinergic input to thecolon, an exaggerated motor response to somepeptidescan develop.