The mechanism of action of gastrin on pig parietal cells was investigated. The aminopyrine accumulation technique was used to estimate acid production in gastric mucosal cells, containing 10-20% parietal cells, and in enriched parietal cells, containing 65-95% parietal cells. The gastrin analogue pentagastrin stimulated aminopyrine accumulation in a dose-dependent fashion irrespective of the proportion of non-parietal cells present. The apparent EC50 for pentagastrin was 5 nM and the maximally effective concentration was 100 nM. The histamine H2-receptor antagonist ranitidine did not affect the action of pentagastrin. The stimulatory effects of various doses of histamine on aminopyrine accumulation in highly enriched parietal cells were potentiated by the inclusion of 100 nM pentagastrin in the incubation medium. In another series of experiments using mucosal cells, the action of effective doses of pentagastrin were potentiated by the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX), which alone elicited an aminopyrine accumulation equal to 50% of that obtained by 100 microM histamine. When ranitidine (100 microM) was included, the action of IBMX was almost completely abolished. However, the dose-response curve for pentagastrin in the presence of ranitidine plus IBMX was similar to that obtained in the absence of IBMX. Dibutyryl-cAMP (DBcAMP, 1 mM) in the presence of ranitidine (100 microM) also potentiated the action of all effective doses of pentagastrin on mucosal cells. The protein kinase A inhibitor Rp-cAMPS, present at 500 microM in the incubation medium, significantly reduced the action of each effective concentration of pentagastrin on aminopyrine accumulation in enriched parietal cells. These results in pig parietal cells were interpreted as indicative of: (i) an action of gastrin exerted directly on the parietal cells; (ii) elevation of intracellular cAMP having a permissive role in the action of gastrin on aminopyrine accumulation.