The human colon can dilate, often to life-threatening proportions. We explored nitrergic mechanisms underlying colonic dilation in conscious dogs with enterically isolated ileocolonic loops either extrinsically innervated (n=4) or extrinsically denervated (n=4). We recorded phasic pressures in ileum and ileocolonic sphincter (ICS), and colonic tone, compliance, and relaxation during ileal distention. By NADPH-diaphorase histochemistry, we assessed effects of extrinsic denervation and enteric isolation on nitrergic fibers. Extrinsic denervation increased phasic pressures in ileum, ICS and colon, and abolished ICS and colonic relaxation in response to ileal distention. The NOS inhibitor L-NNA increased phasic pressures at all sites and ICS tone, but did not abolish colonic relaxation during ileal distention in innervated loops. L-NNA reduced compliance and induced colonic high-amplitude propagated contractions (HAPCs) in denervated loops. The NOS substrate donor L-arginine reversed effects of L-NNA. The number of NADPH-diaphorase fibers increased in both enterically isolated preparations. Non-nitrergic extrinsic nervous pathways mediate reflex colonic relaxation during ileal distention. Enteric isolation augments number of NOS fibers, an effect not modified by extrinsic denervation.