The effects of prostacyclin (PGI2) on gastric emptying and secretion were studied in five unanesthetized chair-adapted rhesus monkeys. A dye dilution technique was used to determine simultaneously gastric fractional emptying, hydrogen ion (H+) output, fluid output, and H+ concentration of the gastric juice. A continuous intravenous infusion of either sodium carbonate buffer (control) or PGI2 (125, 175, or 250 ng/kg/min) was given during a 40-min basal period and following the intragastric administration of an 80-ml water load. During the basal period, 175 and 250 ng/kg/min significantly decreased H+ output compared to control. Only 250 ng/kg/min, however, significantly reduced basal fluid output compared to control. As a result, the H+ concentration of the gastric juice was lowered by both 175 and 250 ng/kg/min. The effects of PGI2 on H+ output, fluid output, and H+ concentration of the secreted juice after the water load were similar to those observed during the basal period. In addition to its anti-secretory action, PGI2 inhibited significantly postload, but not basal, gastric fractional emptying. Because a fall in intragastric H+ concentration is known to enhance fractional emptying, it appears that the retarding effect of PGI2 on fractional emptying is not mediated by its ability to suppress H+ output.