To study the effect of intravenously given omeprazole on the 24-hour gastric secretion, 9 male patients with previously endoscopic proven duodenal ulcer disease were studied. Two four-dose regimens only differing in loading doses were compared. The study showed that 40 mg omeprazole, administered intravenously 4 times a day, is able to keep the gastric pH above 4 in 85% of the 24-hour study period and in 65% of the time pH was above 6. The corresponding values from the baseline period were 10 and 6%. There was no additional effect by a larger loading dose.