The oxidation of sparteine and mephenytoin was examined in a group of subjects living in Greenland: 300 in East Greenland and 171 in West Greenland. The distributions of the ratio between the chromatographic peak areas of S- and R-mephenytoin in the urine, the S/R ratio was clearly bimodal in both populations. Thus 9.3% of the East Greenlanders had S/R ratios of 0.9 or more and were phenotyped as poor metabolizers of mephenytoin. In the West Greenlanders, 2.9% of the sample had S/R ratios of 0.90 or more and were accordingly phenotyped as poor metabolizers. The intraethnic difference with regard to the frequency of the mephenytoin poor metabolizer is probably attributable in part to a much higher proportion of admixed Caucasian genes in the West Greenlanders than in the East Greenlanders. In both the East and the West Greenlanders, the sparteine metabolic ratio displayed marked interindividual differences without a dear bimodal distribution. Poor metabolizers arbitrarily defined as subjects with an metabolic ratio of 20 or more made up 3.3% of the East Greenlanders and 2.3% of the West Greenlanders, but the difference between the two groups was not statistically significant.