A high correlation was observed between the aryl hydrocarbon hydroxylase activities in short-term lymphocyte cultures of 23 individuals and their plasma half-lives of antipyrine and phenylbutazone. Individuals with low inducibility of aryl hydrocarbon hydroxylase activities had very long plasma half-lives of antipyrine and phenylbutazone, whereas subjects with high inducibility of aryl hydrocarbon hydroxylase activites had relatively short plasma half-lives. Individuals with intermediate aryl hydrocarbon hydroxylase activities displayed intermediate half-lives for both drugs. The observed correlation indicates determinants which are common to the elimination of antipyrine or phenylbutazone, and aryl hydrocarbon hydroxylase metabolism of hydrocarbons. The differences in rates of drug elimination are probably due to genetic differences and may have pharmacological and therapeutic significance.