A newly developed prostaglandin E2 derivative, enprostil (TA/RS-84135), was administered to five healthy volunteers in the stomach and duodenum through a teflon tube to examine its effect on amogastrin-stimulated gastric acid secretion. The acid output 1-3 h after administration of enprostil (35 micrograms) decreased remarkably as compared with that in the same period after placebo administration. However, no significant difference in antisecretory effect was observed between the two routes of administration. A decrease in pepsin output occurred in parallel with the decrease in acid output. These results suggest that the antisecretory effect of enprostil is at least partially due to systemic absorption and the predominantly topical action seen in animals does not seem to occur in man. No side-effects attributable to enprostil were observed during the test period. In conclusion, enprostil seems to be clinically useful as an antiulcer agent.