Since some H2-receptor antagonists, like cimetidine or ranitidine, affect ethanol metabolism by interference with gastric and/or hepatic alcohol dehydrogenase (ADH) it was investigated whether omeprazole has a similar effect and its effects were compared with those of cimetidine, an inhibitor of gastric ADH. The first-pass metabolism (FPM), quantified by measuring the difference between areas under the curve (AUC) of ethanol blood concentrations after oral intake or intravenous administration of the same amount (0.3 g kg-1 b.w.) of ethanol (EtOH), was studied before and after 1 week of omeprazole (20 mg daily) or cimetidine (800 mg daily) administration in 10 normal male volunteers. ADH activity was determined in gastric mucosal samples, collected during endoscopy, before and after 1 month of omeprazole treatment. The effect of the drugs on gastric and hepatic ADHs was studied in vitro in both rat and man. No significant effect of omeprazole was found on AUCs of the blood EtOH concentrations. The ADH activity in antral mucosa before and after omeprazole therapy did not show significant differences. In vitro, omeprazole reduced the activity of the low Km gastric ADH with a Ki of 5.6 mM in rat and the hepatic ADH activity with a Ki of 2.4 mM in man, whereas the drug did not show any effect on hepatic ADH in rat and gastric ADH in man. On the contrary, cimetidine increased the AUCs of EtOH blood concentrations after both gastric and intravenous route and, in the in vitro assay, inhibited gastric and hepatic ADH in both man and rat. These results indicate that omeprazole does not affect EtOH metabolism in man and seems to be safer than cimetidine in subjects unable to reduce ethanol intake during the therapy for peptic ulcer or other hypersecretory conditions.