Prostacyclin, its stable metabolite 6-keto-prostaglandin F1αand prostaglandin E2 were tested on the adenylate cyclase system in human gastric mucosa. All three compounds were able to activate human cyclase. Saturating concentrations of prostacyclin, prostaglandin E2 and 6-keto-prostaglandin F1α (each 0.28 mM) increased the enzyme activity 3.0 and 1.6 fold, respectively. The concentrations of prostacyclin and prostaglandin E2 required to produce half maximal activation were in the same range. Prostacyclin is an efficient stimulator of human gastric mucosal adenylate cyclase.