In conscious sheep, tetragastrin, pentagastrin, and synthetic human gastrin I, injected either subcutaneously or intravenously in doses of 156-5,200 pmol/kg body wt, inhibited the vagally dependent cyclical motility of the reticulum and rumen, whereas in vitro pentagastrin (10(-12) to 10(-6) M) had no demonstrable inhibitory or excitatory effects on intrinsically active or quiescent muscle of the reticulum, rumen, and omasal leaves. In vitro pentagastrin (10(-18) to 10(-4) M) stimulated quiescent and intrinsically active longitudinal and circular muscles of the body of the omasum and the body and antrum of the abomasum and potentiated contractile responses of antral muscle to electrical stimulation of intramural cholinergic nerves. Responses in the presence of hexamethonium, atropine, and tetrodotoxin indicated that the excitatory effects on mixed nerve-muscle preparations of omasal and abomasal tissue were mediated both through stimulation of cholinergic neurones and by direct actions on the muscle. In vitro the ovine stomach shows marked regional differences in both its response and sensitivity to gastrin peptides, and their inhibitory effects on reticuloruminal motility in vivo appear to be other than direct.