A comparative study of cholinergic and adrenergic mechanisms was performed on isolated circular muscle strips of the canine ileum, ileocolonic junction and colon. Cholinergic agonists evoked atropine sensitive contractions, which were most prominent in the colon. The presence of neostigmine to inhibit acetylcholinesterase did not alter the relative sensitivities of the tissues to acetylcholine. The different maximal contraction to cholinergic agents discriminates the colon from the ileum and the ileocolonic junction. Alpha adrenoceptor agonists produced greater contractions in the ileocolonic junction and only marginal contractions in the colon. Phentolamine completely and prazosin and yohimbine partially blocked the response of the ileocolonic junction to norepinephrine, suggesting the involvement of both alpha-1 and alpha-2 adrenoceptors. As in other sphincteric gut smooth muscle, the contractile effects of alpha adrenergic agents are more important than the inhibitory ones. The different effects of cholinergic and adrenergic agents on the ileocolonic junction, as compared sphincteric properties in the canine ileocolonic junction. sphincteric properties in the canine ileocolonic junction.