BRL 24924, a specific 5-hydroxytryptamine type 3 (5-HT3) receptor antagonist, was evaluated for effects on gastric secretion of acid and pepsin and possible influences on the effects of serotonin on gastric secretion. Experiments were carried out in conscious dogs with a gastric fistula during a background stimulation of gastric secretion by continuous infusions of pentagastrin, bethanechol or histamine. During infusion of pentagastrin or histamine, BRL 24924, by itself, influenced gastric secretion with stimulation during a low potent background stimulation and inhibition during a potent background stimulation. A serotonin-counteracting effect of BRL 24924 on gastric secretion was found only during infusion of pentagastrin. The secretory stimulation attained by BRL 24924 could be blocked by atropin suggesting a cholinergic mechanism – 5-HT4 receptors? The inhibitory effects on gastric secretion and the serotonin-counteracting effects of BRL 24924 are supposed to be via 5-HT3 receptor