In this investigation, an isolated, perfused rat stomach system was used to elucidate the roles of histamine, serotonin, and the action of cimetidine, methysergide, and propranolol in relation to the in vivo and in vitro administration of compound 48/80. While histamine administered both in vivo and in vitro stimulated acid secretion in the perfused rat stomach, serotonin, added in vitro, inhibited histamine-induced gastric acid secretion. Cimetidine, given either in vivo or in vitro, blocked histamine-induced acid secretion, and methysergide, but not propranolol, reversed the serotonin-induced inhibition of histamine-stimulated acid secretion. Compound 48/80, given in vitro, caused gastric acid secretion that was blocked by pretreatment with cimetidine. Administered in vivo, however, compound 48/80 inhibited both basal and histamine-stimulated acid secretion. This inhibition was partially reversed by pretreatment with methysergide. The absence of inhibition of acid secretion by in vitro-administered compound 48/80 may be related to the timing of the serotonin effect. When serotonin was given prior to histamine, it blocked acid secretion, whereas no inhibition occurred when serotonin was administered together with histamine. None of the other agents investigated affected basal acid secretion. © 1991 Plenum Publishing Corporation.