Patterns of associated alterations in N-deethylation, 3-hydroxylation, and aromatic methylhydroxylation of a single substrate, lidocaine in liver microsomes from 0.7 - 28 months old Fischer 344 rats were examined. These three patterns were all different from one another. In addition to this position selectiveness, a clear sex difference was observed in the pattern of alteration in N-deethylation. These results are consistent with the hypothesis that age associated alterations in drug metabolism are caused by age associated alterations in relative abundance of multiple species of cytochrome P-450.