Omeprazole induces hepatic cytochrome P-4501A2. In a previous study this effect was shown to be significant in vivo in 6 poor metabolizers, including 1 intermediate metabolizer, but not in 12 extensive metabolizers of S-mephenytoin after 7 days of treatment with 40 mg/day omeprazole. In this study, the specificity of the inducing potential of omeprazole was investigated in these volunteers. Furthermore, in eight of the extensive metabolizers the dose-dependence of cytochrome P-450 1A2 induction was evaluated. Cytochrome P-450 1A2 activity was monitored by means of the '3C-[N3-methyl]caffeine breath test and by means of plasma caffeine clearance before omeprazole treatment with 120 mg/day, on the seventh day of dosage and after a 7-day washout. Omeprazole plasma concentration was measured. Results were compared with those after 40 mg. y-Glutamyltransferase activity in serum, as well as urinary excretion of D-glucaric acid and 6p-hydroxycortisol, were measured on the same study days in all study groups (n = 26). In the eight extensive metabolizers the breath test indicated a dose-dependent increase of cytochrome P-450 1A2 activity of 8.5% f 15.0% (40 mg, mean * SD, NS) and 27.2% 2 16.5% (120 mg, p = 0.002). Caffeine clearance was increased by 31.6% f 20.7% (p < 0.001) with the higher dose. None of the study groups exhibited a significant increase of y-glutamyltransferase activity or urinary excretion of D-glucaric acid or 6P-hydroxycortisol. This was in contrast to the phenobarbital-type induction observed after treatment with antiepileptic drugs. Induction by omeprazole seems to be restricted to cytochrome P-450 1A enzymes. Thus cytochrome P-450 1A2 induction is not clinically relevant with common therapeutic doses in EMS but might be of