Aging results in a significant decline in liver drug metabolism which is largely attributable to changes in the microsomal mixed function oxidase system. For example, the mixed function oxidase system in the livers of senescent rats is characterized by: (1) a reduced cytochrome P-450 content; (2) a decline in the specific activity of NADPH-cytochrome c (P-450) reductase; and (3) a slower rate of ethylmorphine N-demethylation in comparison to young adult animals. Since several factors intrinsic to the microsomes may influence the efficacy of the mixed function oxidase system, e.g. the phospholipid and cholesterol contents, the saturation index of the fatty acids and the fluidity of the membranes, we conducted a physicochemical analysis of liver microsomes isolated from young adult (3-4 months), mature (12-16 months) and senescent (25-27 months) male Fischer rats. Although the microsomal cholesterol content did not change appreciably between maturity and senescence, there was a marked decline in the total phospholipid content. This resulted in a significant increase in the cholesterol/phospholipid ratio, 0.49 to 0.65 between 16 and 27 months of age. The age-related changes in the total phospholipid content were largely reflected in each of the major fractions, i.e. phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine + phosphatidylserine. Small increases in the relative percentages of highly unsaturated fatty acid species were offset by similar decreases in the more frequent and more saturated species as a function of increased age. As a result, the net change in the fatty acid saturation index was probably minimal. However, the increase in the cholesterol/phospholipid ratio most likely contributes to the significant decline in the order parameter of microsomes isolated from old rats which, in turn, may impair the functional capacity of the hepatic mixed function oxidase system.