Postmortem changes in the drug-metabolizing enzymes in rat liver microsome were studied. Parameters investigated were: microsomal protein, NADPH-cytochrome P-450 reductase activity, NADH-cytochrome b5 reductase activity, cytochrome b5 content, cytochrome P-450 content, aminopyrine N-demethylase activity, aniline p-hydroxylase activity, p-nitroanisole O-demethylase activity, uridine diphosphate (UDP)-glucuronyl transferase activity and glutathione S-transferase activity. Nearly all the parameters based on microsomal protein decreased during autolysis and the time-dependent decrement ratios of the parameters changed by various amounts. Cytochrome b5 content decreased more rapidly than that of other components. By 36 h post mortem, levels of cytochrome b5 were not detectable. By 48 h post mortem, NADPH-cytochrome P-450 reductase activity decreased to 91%, NADH-cytochrome b5 reductase activity decreased to 94%, and cytochrome P-450 content decreased to 92% of relative activities. By 48 h post mortem, aminopyrine N-demethylase activity decreased to 87%, aniline p-hydroxylase activity decreased to 98% and p-nitroanisole O-demethylase activity decreased to 75% of relative activities. The activity of p-nitroanisole O-demethylase appeared to be more stable than that of aminopyrine N-demethylase or aniline p-hydroxylase. These results demonstrate that there are multiple forms of isozymes of the cytochrome P-450-linked monooxygenase system. Hepatic transferases showed decrease patterns different to those of monooxygenases, so UDP-glucuronyl transferase activity of approximately 32% of relative activity was detected at 48 h post mortem. Thus, UDP-glucuronyl transferase activity appeared to be more stable than the cytochrome P-450-linked monooxygenases. These results show that these activities and components would be useful as markers of postmortem time. The causes of the variety of instability of these enzyme systems are discussed.