Tolbutamide hydroxylation has been investigated in human liver microsomes. Anti human liver NADPH-cytochrome P-450 reductase IgG inhibited hydroxytolbutamide formation and this metab olite was not formed when NADPH-generating system was omitted from microsomal incubations. Tolbutamide hydroxylation followed Michaelis-Menten kinetics, consistent with the involvement of a single form of cytochrome P-450 in this reaction. Mean apparent Km, and Vmax values for hydroxy tolbutamide formation were 120 ± 41 μM and 0.273 ± 0.066 nmol min-1 mg-1, respectively. A range of clinically used drugs and xenobiotics used as probes for cytochrome P-450 activity in laboratory animals was screened for inhibitory effects on hydroxytolbutamide formation. Caffeine, paraxanthine, theophylline, theobromine, debrisoquine, erythromycin, phenacetin, propranolol, amino pyrine, benzo(a)pyrene and 7-ethoxycoumarin were all round not to inhibit tolbutamide hydroxylation. In contrast, sulphaphenazole, phenylbutazone, nifedipine, verapamil, cimetidine, aniline, dextro propoxyphene and mephenytoin were competitive inhibitors of tolbutamide hydroxylation. The respective apparent Ki values for these compounds were 0.12 μM, 11 μM, 15 μM, 118 μM, 140 μM, 182 μM, 225 μM and 375 μM. Sulphinpyrazone inhibited tolbutamide hydroxylation with atypical kinetics. The in vitro data is in good agreement with in vivo drug interactions with tolbutamide. The data also confirm that tolbutamide hydroxylation is not associated with the cytochromes P 450 responsible for methylxanthine metabolism or with the form responsible for the polymorphic oxidation of debrisoquine.