Serum gastrin levels were determined in 120 consecutive patients (43 females, 77 males) with peptic disease 24 h post dose after 4 weeks continuous omeprazole (40 mg daily) treatment. Serum gastrin levels were elevated in 33 (28%) but exceeded the twice normal range in only 6 patients (5%). Age and sex did not influence the magnitude of gastrin levels. Gastrin increments induced by omeprazole compared to pretreatment with H2-blockers in 60 cases were similar in both males (42 +/- 10 pg/ml) and females (44 +/- 9 pg/ml). It is concluded that the magnitude of gastrin increases observed during omeprazole therapy are small and independent of age and sex.