The ability of gastrin to stimulate acid formation was studied in gastric glands and isolated parietal cells obtained from rabbit gastric mucosa. Accumulation of the weak base aminopyrine and increases in oxygen consumption were used as measures of acid secretory activity. The responses to gastrin were found to be very small (10-15% increase). However, inclusion of dithiothreitol (0.5 mM) in the incubation medium enhanced the responses in both glands and isolated cells to easily detectable levels. For the gastric gland preparation, gastrin stimulation was maximal at 1 X 10(-7) M, with an apparent ED50 of 5 nM. The response reached a maximum at about 30 min and was stable for at least an hour. The gastrin response was enhanced by the phosphodiesterase inhibitor isobutylmethylxanthine and partially inhibited by cimetidine, a histamine H2-receptor antagonist. Combinations of gastrin and histamine showed an additive response over a wide range of histamine concentrations. However, time-course studies revealed a transient potentiation of gastrin response by histamine, which reached a peak at 15 min and was reduced to an additive response by 30 min. Studies using isolated cell populations enriched in parietal cells (approximately 70%) revealed a gastrin stimulation that was not inhibited by cimetidine. The transient potentiation of the gastrin response by histamine was also found in the isolated cell preparation. Gastrin had no effect on cellular cAMP levels or adenylyl cyclase activity. The results are interpreted to indicate that gastrin stimulates acid secretion through three separate actions: 1) a direct stimulation of parietal cell activity, 2) a potentiating interaction with histamine, and 3) for more intact preparations, a release of histamine, which in turn acts as a paracrine stimulus. Quantitatively, the most important action appears to be the release of histamine. None of the actions of gastrin appear to involve a change in cAMP metabolism.