Pulmonary microsomal polypeptides from different strains of rats were resolved using two-dimensional electrophoresis and were further characterized by in situ peptide mapping. Triton X-114 detergent separation was used to enrich cytochromes P-450 (P-450) and other integral membrane proteins from pulmonary microsomes, and these were directly compared with corresponding polypeptides from hepatic microsomes. The results demonstrated that P-450b and epoxide hydrolase were present in the lungs of male and female rats and that their expression in this tissue was independent of phenobarbital treatment. P-450e, which is co-induced with P-450b in the liver, was not detected in pulmonary microsomes under any condition. Four other pulmonary microsomal polypeptides were characterized and preliminary evidence suggested that they represent unique isozymic forms of P-450 with three of them being related to P-450b.