The actions of two mast cell stabilising agents, FPL 52694 and disodium cromoglycate and the Ca2+ channel antagonist verapamil on acid secretion by the rat isolated stomach have been studied. FPL 52694 (1-5 mM) in the serosal medium stimulated acid secretion. This was antagonised by prostaglandin E2 but not by atropine, metiamide, propranolol or tetrodotoxin. FPL 52694 (1-5 mM) in the mucosal medium had an inhibitory effect on basal acid secretion and that stimulated by pentagastrin and histamine. The inhibition reversed only slowly after removal of FPL 52694. Disodium cromoglycate and serosal application of verapamil had no significant effects on acid output. Removal of calcium from the bathing solutions and mucosal verapamil inhibited basal secretion. Topically applied, FPL 52694 is a potent inhibitor of acid secretion in vitro. This effect may involve an action upon the availability of calcium to the tissue. The stimulatory effects of serosal FPL 52694 are discussed in relation to previous in vivo findings.