Treatment of rats with p-chlorotoluene (PCT, 1 g/kg, ip) resulted in peak PCT blood and lung concentrations at 4 h, which declined to very low levels at 12 h. The concentration of PCT in the liver attained its highest value at 1 h and also declined to low levels at 12 h. In the dose-response study, PCT significantly decreased hepatic and pulmonary AHH activities at 0.5 g/kg, 1 h. Maximum inhibition was attained at 1 g/kg, 1 h, and further increase in the dose did not enhance the enzyme inhibition in the time-course investigation, PCT (1 g/kg) maximally inhibited hepatic and pulmonary aryl hydrocarbon hydroxylase (AHH) activities at 1 h and the decrease in enzyme activity was sustained through 12 h. Administration of PCT (1 g/kg, 1 h) also markedly decreased pulmonary cytochrome P-450 content, while hepatic cytochrome P-450 content was only slightly reduced. The partial decrease in cytochrome P-450 content indicated altered levels of the P-450 isozymes, which may have profound effects on the metabolic disposition of benzo[a]pyrene [BaP]. BaP is regioselectively metabolized by two major isoforms of P-450 to toxic dihydrodiols and nontoxic phenol derivatives and there is a balance between these two metabolite groups. PCT (1 g/kg, 1 h) significantly inhibited the phenolic 3-OH BaP formation in both lung (52%) and liver (56%). The formations of BaP 7,8-dihydrodiol (146%) and 9,10-dihydrodiol (90%) were significantly elevated in the lung. The toxication to detoxication ratios were significantly elevated in both organs. Total quinone formation was markedly enhanced in the liver. Since PCT inhibited phenolic metabolite formation and increased dihydrodiol production, the activities of the isozymes that are responsible for their formations were determined. PCT (1 g/kg, 1 h) significantly inhibited cytochrome P-4502B1 in the lung (50%) and 2B1/2B2 in the liver (40%), while cytochrome P-4501A activity was not altered in either lung or liver. PCT increased phospholipid (PL) levels (45%) and conjugated diene (CD) formation (58%) in lung but not in liver, while membrane fluidity was increased [phospholipid/cholesterol (PL/CL) ratio; diphenylhexatriene (DPH) and trimethylammonium DPH (TMA-DPH) fluorescence polarization] in both organs. There was no apparent relationship between these membrane changes and alterations in MFO activity. Taken together the results suggest that PCT is capable of nonselectively inactivating the hepatic and pulmonary isozymes of P-450 that are responsible for the detoxication of BaP. The observed shift in the metabolism of BaP toward potentially more toxic metabolites suggests that concurrent exposure to BaP and PCT may result in greater toxicity, compared to exposure to BaP alone.