In vitro studies on the vascularly perfused rat stomach have shown that gastrin secretion is regulated by intramural cholinergic and noncholinergic neurons. We have postulated that bombesin (gastrin-releasing peptide), a known gastrin stimulant present in antral mucosal nerve fibers, is the most likely candidate for noncholinergic transmitter. Bombesin antiserum (final dilution, 1:150) but not control serum added to the vascular perfusate inhibited the gastrin response to 1,1-dimethyl-4-phenylpiperazinium by 59 +/- 17% (P less than 0.01) and to electrical field stimulation by 60 +/- 16% (P less than 0.01), and its effect was additive to that of 10(-7) M atropine (75-94%), thus accounting for the greater part of neurally induced gastrin secretion. The effects of atropine and bombesin antiserum on somatostatin secretion were consistent also with blockade of cholinergic and noncholinergic neurons, respectively. The results indicate that bombesin and acetylcholine are the main intramural neural regulators of gastrin and somatostatin secretion. Acetylcholine acts predominantly to decrease the paracrine secretion of somatostatin, thereby eliminating the continuous restraint of somatostatin on gastrin secretion and enabling bombesin to exert its potent stimulatory effect on gastrin secretion.