Adult female Wistar rats (n = 48) divided into four groups of 12 were treated orally with 3 mg/kg per day thalidomide, a 0.05% hexachlorobenzene (HCB)--containing diet, with both drugs together and with the vehicles (controls) over periods of 10 and 60 days. The protein and P450 contents and the activities of aminopyrine-N-demethylase (ADM) and 7-ethoxyresorufin-O-deethylase (7-ERO-D) were determined in the liver microsomes. The activity of 7-ERO-D was also determined in the skin microsomes and total porphyrins were measured in the urine of the animals. Thalidomide increased the hepatic P450 content, caused distinct changes in the activities of the hepatic and cutaneous microsomal isozymes, modified their induction by HCB and inhibited the porphyrogenic activity of HCB. These findings indicate an interaction between thalidomide and HCB with regard to their effects on the P450 isozymes and the metabolism of porphyrins.