A study of the in vitro metabolism of N,N-dimethylaniline using guinea pig and rabbit preparations and GLC techniques has confirmed N-demethylation and N-oxidation and established ring hydroxylation as metabolic routes. Whereas N-demethylation and N-oxidation are major routes of metabolism, ring hydroxylation is a comparatively minor pathway. Like N-demethylase and N-oxidase, the 4-hydroxylase has been shown to be a microsomal enzyme. The major ring hydroxylated product of N,N-dimethylaniline is N,N-dimethyl-4-aminophenol; N-methyl-4-aminophenol is subsequently formed. The apparent Km and V max values for N-demethylation, N-oxidation and 4-hydroxylation, are presented for both the guinea pig and the rabbit.