Plaice were treated with an acute dose of a polyaromatic hydrocarbon (3-methylcholanthrene, 3-MC) or cadmium, or 3-MC and cadmium by i.p. injection. The effects on hepatic detoxication systems, cytochrome P-450 (ethoxyresorufin O-deethylase, EROD), UDP-glucuronyl transferase, glutathione S-transferase, glutathione peroxidase activities, total glutathione (GSH), metallothionein and Cd and Zn in the cytosol were studied over a 14 day period. 3-MC increased EROD (7-18-fold), glucuronyl transferase (40%) and GSH transferase (200%) activities, whereas GSH peroxidase activity decreased by 60%. Cd treatment inhibited EROD (90%), GSH transferase (90%) and GSH peroxidase (30%) activities and displaced Zn. Total GSH levels increased (200%) prior to onset of metallothionein synthesis (6 days). Cotreatment with 3-MC and Cd led to a marked increase in GSH levels (300%) but the onset of metallothionein synthesis was delayed by a week. Induction of enzyme activities was abolished, EROD activity was strongly inhibited and there was a transient 50-90% decrease in glucuronyl transferase, GSH transferase and GSH peroxidase activities on days 2 and 3 after treatment. The results indicate that a polyaromatic hydrocarbon could result in increased peroxidative damage, the heavy metal Cd can severely inhibit organic xeno- and endobiotic metabolism and that the effects of both agents may be synergistic.