Nutritional-pharmacological interactions were studied in a group of malnourished subjects. Antipyrine was used to evaluate mixed-function oxidase in man. The results indicated that the rate of disappearance of antipyrine from plasma was strongly influenced by the nutritional status of the individual. The half life of antipyrine was modified in undernourished subjects and those with nutritional oedema. This finding indicates that drug regimens may have to be adjusted in patients who have antipyrine half lives that are shorter or longer than normal. Otherwise drug treatment may be inadequate or, in patients with impaired microsomal enzyme activity, potentially dangerous.