The influence of in vivo administration of detergents on the hepatic microsomal cytochrome P-450 system was studied in rat. Male Wistar rats were administered detergents, Emulgen 913 (50 mg or 100 mg kg-' of body weight (B.W.)), or sodium dodecylsulfate (SDS, 25 mg or 50 mg kg-' of B.W.) intraperitoneally once a day for 3 days. Cytochrome P-450 content in liver microsomes was significantly decreased to 85% and 73% of control by the administration of 50 mg or 100 mg Emulgen, respectively, but the microsomal protein concentration was not changed by these administrations. The content of cytochrome P-450 was also reduced to 76% and 70% of control by the administration of 25 mg or 50 mg SDS/kg of B.W., respectively. The total hydroxylation activity (the sum of o-and (o -1)-hydroxylase activity) of laurate almost paralleled the decrease in cytochrome P-450 in detergent-treated rats. However, the w/o-1-hydroxylation ratio was not changed. These results suggest that the administration of these detergents lowered the level of cytochrome P-450 species catalyzing o-and (o -1)-hydroxylation of laurate to a similar extent. On the other hand, aminopyrine Nand p-nitroanisole 0-demethylation activities in Emulgen 913-treated rats was decreased while those in SDS-treated rats did not change, though the content of cytochrome P-450 was decreased by both administrations. Thus, it was demonstrated that the livers of rats responded to exogenous detergents in different manners.