This study is concerned with an investigation of the hypothesis that metoclopramide enhances the contractile activity of the guinea pig antrum by increasing acetylcholine release from the postganglionic cholinergic nerve ending. Longitudinal muscle strips were stimulated repetitively (200 microseconds pulses, 20 Hz, supra maximal current) for 2 hr in the presence of hemicholinium-3 (200 micron). Pretreatment in this manner produced a mean reduction of 50% in the acetylcholine content, when compared with that in repetitively stimulated control strips not incubated with hemicholinium-3. In the hemicholinium-3-treated strips the normal excitatory response to metoclopramide (60 micrometer) was prevented. Only a small reduction in response to metoclopramide was detected in strips which were incubated with hemicholinium-3 but not stimulated repetitively, and in which there was no significant change in acetylcholine content. The stimulant effect of metoclopramide depends, therefore, upon maintenance of intrinsic stores of acetylcholine.