Abstract:The inducing effects of octachlorostyrene (OCS) and hexachlorobenzene (HCB), chlorinated hydrocarbon contaminants found in fish, on the microsomal cytochrome P‐450 system were studied in rats. Administration of single doses of OCS and HCB at 50mg/kg intraperitoneally and higher led to increases in microsomal protein and cytochrome P‐450 content, cytochrome P‐450 reductase, ethylmorphine N‐demethylase, 4‐nitroanisole O‐demethylase, acetanilide 4‐hydroxylase, but not in benzo(a)pyrene hydroxylase activities. Similar patterns of induction were seen after 14 days repeated intraperitoneal or oral dosing. Sodiumdodecylsulfate polyacrylamide gel electrophoresis of microsomes from OCS and HCB treated rats showed increases in proteins similar to those found after phenobarbital (PB) induction. Under the present experimental conditions OCS and HCB were found to be PB