Effect of mosapride, a benzamide, on the motor activity associated with the release of endogenous acetylcholine (ACh) from enteric neurons was examined in the ileum of anesthetized dogs using an in vivo microdialysis method and compared with the effect of 5-hydroxytryptamine (5-HT). Intraarterial administration of 5-HT accelerated intestinal motor activity and increased the concentration of dialysate ACh, and the responses were inhibited by SB204070, a specific 5-HT₄-receptor antagonist, but were apparently not affected by methiothepin, ketanserin and granisetron. Intraarterial administration of mosapride, a prokinetic benzamide, accelerated intestinal motor activity and the concentration of dialysate ACh increased. The effects of mosapride were antagonized by SB204070. Specific [¹²⁵I]SB207710 binding was observed in the myenteric and submucosal plexuses and muscle layers of dog ileum by in vitro receptor autoradiography. High densities of [¹²⁵I]SB207710 binding sites were detected in the myenteric and submucosal plexuses. Mosapride as well as SB204070 inhibited [¹²⁵I]SB207710 binding. Thus, in the whole body of dogs, 5-HT and mosapride accelerated the intestinal motor activity due to the increases in ACh release mediated by stimulation of the 5-HT₄ receptor.