The study was aimed at defining the relationships among the oxidative capacities for three prototype drugs, metoprolol, debrisoquine and sparteine, used for assessing genetically determined polymorphism of drug oxidation in a Japanese population. Among 292 unrelated healthy Japanese subjects who had been defined as extensive (EMs, n = 291) or poor (PM, n = 1) metabolisers of metoprolol oxidation, 55 subjects (EMs = 54 and PM = 1) were selected. One PM of metoprolol oxidation was also identified as a PM not only of debrisoquine but also of sparteine, and no misclassification by the three phenotypic methods was observed. All three correlations among the metabolic ratios of the three test probes assessed by Spearman's rank test were highly significant (P < 0.001). These findings indicate that in Japanese subjects the oxidation capacities of metoprolol, debrisoquine, and sparteine are closely related. It appears that in Japanese the polymorphic oxidation of the three drugs is co-regulated, either by the same enzyme or gene-controlling system.