The effects of seven days' treatment with omeprazole 5 and 10 mg daily on 24 hours gastric secretion and plasma gastrin concentrations were studied in a randomised double-blinded placebo-controlled study of six male patients with healed duodenal ulcer. Omeprazole 5 mg daily reduced mean daytime and nocturnal intragastric acidity by 31.4 and 40.1%, respectively. Omeprazole 10 mg per day produced very similar reductions of 33.6 and 42.0%, respectively. Total nocturnal acid output was reduced by 63.9% and 63.2%, respectively, by omeprazole 5 and 10 mg daily. There was a large degree of inter-subject variability in response to these low doses of omeprazole. Consequently, neither dose showed a statistically significant antisecretory effect when compared with placebo. Neither dose of omeprazole significantly affected fasting levels of gastrin, but omeprazole 10 mg daily produced a significant (P less than 0.05) increase in the integrated gastrin response to a meal. The lack of consistent antisecretory effect to low dose omeprazole is in accord with previous studies. This suggests that doses of 20 mg per day or greater are required to produce a consistent effect on acid secretion.