The effect of prostacyclin (PGI2), its chemical decomposition product, 6-oxo-PG[prostaglandin]F1.alpha. and a stable 5,6-dihydro analog, 6.beta.-PGI1, on gastric acid secretion, mucosal blood flow (14C-aminopyrine clearance) and mean arterial pressure in unanesthetized dogs was studied. During submaximal acid secretion from a gastric fistula induced by i.v. histamine dihydrochloride (20 .mu.g/kg per h), PGI2 and 6.beta.-PGI reduced acid output with ID50 (dose causing 50% inhibition) of .apprx. 0.2 and 3.0 .mu.g/kg per min i.v., respectively, but 6-oxo-PGF1.alpha. was inactive at 100 times the effective dose of PGI2. The ratio of mucosal blood flow to acid output remained unchanged during PGI2 administration and was significantly elevated during 6.beta.-PGI1 infusion, suggesting that with both compounds the changes in mucosal blood flow were not the cause of antisecretory action. For doses causing equivalent antisecretory action, 6.beta.-PGI1 lowered systemic arterial blood pressure much less than PGI2, indicating selectivity of action. PGI2 is unlikely to be a circulating antisecretory agent, but may be a local humoral modulator of secretion and blood flow in the gastric mucosa.