The genetic oxidation polymorphisms of debrisoquine and proguanil were studied in a New Zealand Maori population. A bimodal distribution was observed in the 0-4 h urinary debrisoquine/4-hydroxydebrisoquine metabolic ratio. Of 101 Maori subjects phenotyped, five subjects (5%) were identified as poor metabolizers of debrisoquine, according to criteria established in studies of Caucasian populations. The prevalence of the debrisoquine poor metabolizer phenotype in the Maori appears to be similar to that reported for the Caucasian populations, but higher than that found in Asian (non-Caucasian) populations. The distribution of proguanil:cycloguanil (PG:CG) ratios obtained from 43 Maori subjects was highly skewed. Using a PG:CG ratio of 10 as the cut-off point, three Maori subjects (7%) were classified as poor metabolizers of proguanil. The incidence of the poor metabolizer phenotype of proguanil oxidation of 7% seems to be higher in Maori compared with Caucasian populations, but this is lower than the usual ranges (15-35%) reported in Asian populations.