Antipyrine clearance has been measured from serial serum samples in 49 healthy black Africans from a village in Southern Africa. The subjects follow a lifestyle which minimally exposes them to environmental inducing or inhibiting agents. Food is mainly maize cereal with a protein content of only about 8.8%, together with greens. Antipyrine clearance, half-life and apparent volume of distribution (mean +/- SD) were, respectively, 0.538 +/- 0.163 ml min-1, kg-1, 14.81 +/- 6.5 h and 0.626 +/- 0.075 litre/kg. These results do not differ significantly from the mean values found in a group of lactovegetarian Indo-Pakistani immigrants to Britain. This would suggest that the major environmental determinant influencing hepatic mixed-function oxidase activity is the presence or absence of meat in the diet. However, the relative contributions of environment and heredity will be difficult to determine.