Prostaglandins (PG)I 2, PGE 2 and 6-keto PGF 1α were infused directly into the gastric arterial supply at 10 −9, 10 −8 and 10 −7 g/kg/min during an intra-gastric artery pentagastrin infusion in anesthetized dogs. 6-keto PGF 1α was also infused at 10 −6 g/kg/min. Gastric arterial blood flow was measured continuously with a non-cannulating electromagnetic flow probe and gastric acid collected directly from the stomach. PGI 2 and PGE 2 produced similar dose-dependent increases in blood flow with an increase of more than four-fold at the highest dose. Both PGs inhibited acid output over this dose range with PGE 2 having 10 times the potency of PGI 2. 6-keto PGF 1α was at least 1000 times less active than PGI 2 or PGE 2 at increasing blood flow and failed to inhibit acid output even at 10 −6 g/kg/min.