Isolated canine parietal cells were used to evaluate inhibition of acid secretion by both endogenously synthesized and exogenously added prostaglandins (PGs). Uptake of [14C]aminopyrine by the parietal cells was used as an index of acid secretion. The effect of increasing endogenous PG synthesis was determined by incubating arachidonic acid with histamine-stimulated parietal cells. Arachidonic acid (10(-6) and 10(-5) M) was a potent inhibitor of histamine-stimulated acid secretion. This effect of arachidonic acid was totally abolished when the cells were pretreated with 100 microM indomethacin. The effect of exogenous, preformed PGs on histamine-stimulated acid secretion was evaluated by adding PGE2, PGI2, PGF2 alpha or PGD2 to parietal cells. PGE2 was the most potent inhibitor of acid secretion, with a 50% inhibition of maximal acid secretion (ID50) at 7.5 X 10(-8) M. PGI2 and PGF2 alpha also inhibited acid secretion, but at higher concentrations. The ID50 for PGI2 and PGF2 alpha was 10(-5) M. PGD2 was inactive at inhibiting acid secretion. We conclude that both endogenous and exogenous PGs inhibit acid secretion directly at the parietal cells and that PGE2 is most likely the PG produced locally to modulate parietal cell acid secretion.