The effect of 7 days of oral dosing with 5 mg day-1 and 20 mg day-1 omeprazole on basal and pentagastrin-stimulated gastric acid output was studied in nine duodenal ulcer patients. Basal acid output measured 5-6 h post-dosing was decreased by a mean of 75% on 5 mg omeprazole and by 90% on 20 mg omeprazole (P less than 0.05 and P less than 0.01, respectively). Peak acid output measured 6-7 h post-dosing was decreased by a mean of 75% on 5 mg omeprazole and 97% on 20 mg omeprazole (P less than 0.01 for each). There was a wide interindividual variation in response to the 5 mg dose, with five of the nine patients having more than 90% inhibition of peak acid output, but two patients having less than 40% inhibition. This unpredictable response to daily low-dose omeprazole therapy makes it unsuitable for maintenance treatment.