The effect of omeprazole on acid secretion and gastrin levels has been investigated in 10 elderly duodenal ulcer patients in remission. Doses of 5, 10, 20 and 40 mg omeprazole were given once daily for 7 consecutive days and the basal (BAO) and peak (PAO) acid output and fasting plasma gastrin concentration were measured 24 h after the seventh dose. Omeprazole suppressed PAO significantly and dose-dependently after doses of 10, 20 and 40 mg, the suppression being 42%, 75% and 85%, respectively. No patient showed complete inhibition of PAO and at least 20 mg had to be given to obtain a marked inhibitory effect in all patients. Increasing the dose to 40 mg had only a slight additional effect compared to 20 mg. There was a relationship between degree of acid inhibition and the increase in fasting plasma gastrin. PAO had to be suppressed by more than 80% before a moderate increase in fasting plasma gastrin was observed. The optimal once-daily oral dose of omeprazole for inhibition of acid secretion in elderly patients appears to be 20 mg. Omeprazole 20-40 mg may cause a moderate increase in fasting plasma gastrin.