The response of the rat proximal colon to distension and drugs that interfere with intrinsic and extrinsic nerves was investigated in vivo (urethan anesthesia) and in vitro. Saline distension induced the appearance of a cyclic contractile activity that was slightly inhibited by atropine (ATRO) and enhanced by physostigmine. Hexamethonium increased the distension-induced motor activity. Topical tetrodotoxin (TTX), lidocaine, or procaine produced an increase in motility of the proximal colon. Isolated segments of the proximal colon exhibit a high-amplitude phasic contractile activity that was increased by stretching, transiently inhibited by ATRO, unaffected by hexamethonium, and increased by TTX. The effects of both ATRO and TTX were more evident at high- than low-resting tone. In the presence of ATRO plus guanethidine, field stimulation of the isolated rat proximal colon suppressed the spontaneous contractile activity of the preparations. These findings indicate that, in the proximal colon of urethan-anesthetized rats, a tonic discharge of intramural nonadrenergic noncholinergic neurons suppresses the inherent myogenic contractile activity of the smooth muscle cells. Extrinsic nervous supply plays a subsidiary role in maintaining colonic motility.