In 8 volunteer male test subjects having an average age of 25.5 years, the influence of i.v. infused PGE1 on the gastric secretion stimulated with pentagastrin was investigated. Pentagastrin was infused continuously over a period of 2 h at a dose of 2 μg/kg body wt./h. Sixty minutes after the start of the infusion, an additional 460 μg PGE1 (5–7 μg/kg body wt.) in 250 ml saline were infused over a period of 30 min. During the infusion of PGE1 the volume and H ion output of the gastric juice were significantly reduced. The most marked reduction occurred in the time period between 15 and 45 min after the start of PGE1 administration. With these experiments it has been proved for the first time that with i.v. administration of PGE1 as opposed to oral administration, not only the basal but also the stimulated human gastric secretion is inhibited. The side effects in this series of experiments were tolerable.