Previous studies with omeprazole have demonstrated that a daily dose of 30 or 40 mg reduces the acidity of gastric contents throughout the 24-hour period (1, 2). The influence of dosing time has been examined with 40 mg given either in the morning or in the evening (1). A dose at 0900 hours produced a slightly greater overall elevation in 24-hour pH than did dosing at 2100 hours. Inhibition of gastric acidity was nearly complete with either regimen, so that the difference between them was of doubtful therapeutic significance. At a lower dose, which may be considered for maintenance therapy, the time of administration may be more important. Therefore, 24-hour gastric pH profiles were examined in duodenal ulcer patients given omeprazole, 10 mg, in the morning or the evening.