Gastrin 17-1 was infused into conscious rats equipped with jugular and portal vein catheters and with a pyloric gastric drainage tube to achieve serum concentrations slightly higher than those found during intragastrically instilled peptone solutions. I.v. injected rabbit gastrin antiserum abolished gastrin 17-1-stimulated gastric acid secretion in these animals. Gastrin immunoneutralization reduced peptone-stimulated gastric acid secretion by approximately 37% during the entire 30-min period of stimulation. Peptone-stimulated gastric acid secretion was only significantly (P < 0.05) inhibited in the third 10-min period, but not in the first or second 10-min periods of stimulation. This study demonstrates that the late but not the initial period of peptone-stimulated gastric acid secretion is regulated by circulating gastrin in rats.