Omeprazole, a substituted benzimidazole, and a potent inhibitor of gastric parietal cell H+/K+ ATPase, was tested for drug interactions at two dose levels (30 mg and 60 mg/day) in man using the model drugs [14C]-aminopyrine and antipyrine. Elimination of both models was assessed before and after 15 days treatment with omeprazole. In addition [14C]-aminopyrine metabolism was assessed on day 2 of treatment to investigate the rapidity of onset of any effect. In 10 healthy male volunteers omeprazole 60 mg/day for 14 days prolonged aminopyrine 14CO2 half life (t1/2), measured on the 15th day, by 21% (P less than 0.05), and reduced percent dose demethylated in 2 h (ABT2) by 19% (P less than 0.005). No effect was seen on day 2 of therapy. After 14 days treatment antipyrine half-life was prolonged by 10% (P less than 0.025) and clearance was reduced by 14% (P = 0.063). In nine healthy male volunteers omeprazole 30 mg/day for 14 days prolonged aminopyrine 14CO2 by 13% and reduced ABT2 by 11%. Both changes just failed to reach statistical significance. At this dose antipyrine metabolism was unaltered. As the dose of omeprazole used in clinical practice may be less than 30 mg/day it is unlikely that metabolic inhibition will occur during routine use, although it will be necessary to test for interactions with therapeutically more important compounds. Interactions should be looked for when large doses of omeprazole are being used to treat hypersecretory states.