Hepatic monooxy genase activities were studied in microsomal fractions from two species of freshwater fish, the nase (Chondrostoma nasus) and the European roach (Rutilus rutilus). These activities were determined by using four substrates, 7-ethoxycoumarin, 7-ethoxyresorufin, benzo(a)pyrene, and 2,5-diphenyloxazole and were characterized according to pH, temperature, cofactors, and the differential effects of two inhibitors, metyrapone and α-naphthoflavone. Solubilization of microsomes was achieved by the use of detergents, with a good recovery of the cytochrome P-450.