We previously reported that the levels of epoxyeicosatrienoic acids (EETs) and 20-hydroxyeicosatetraenoic acid (20-HETE) are depressed in microsomes prepared from lungs of rats with acute Pseudomonas pneumonia. We also showed a potential role for cytochrome P-450 (CYP) metabolites of arachidonic acid (AA) in contractile responses of both normal pulmonary arteries and pulmonary arteries from rats with pneumonia. The CYP2J subfamily enzymes (endogenous source of EETs and HETEs) are constitutively expressed in human and rat lungs where they are localized in vascular smooth muscle and endothelium. The purpose of this study was to determine if CYP2J proteins are modified in pneumonia. Pseudomonas organisms were injected via a tracheostomy in the lungs of rats. Later (44 h), lungs were frozen, and microsomes were prepared from pneumonia and control rat lung homogenates. Lung microsomal proteins were then immunoblotted with anti-CYP2B1/2B2, anti-CYP4A, anti-CYP2J9pep2 (which reacts with rat CYP2J3), anti-CYP2J6pep1 (which reacts with rat CYP2J4), anti-CYP2J2pep4, or anti-CYP2J2pep3 (both of which react with all known CYP2J isozymes). Western blotting revealed a prominent 55-kDa band with anti-CYP2J2pep3, anti-CYP2J2pep4, and anti-CYP2J6pep1 (but not anti-CYP2J9pep2) that was reduced in pneumonia compared with control lung microsomes. The CYP2B bands (51-52 kDa) were less prominent and not different between pneumonia and control lungs. CYP4A proteins (20-HETE sources) were not detected in rat lung microsomes. Therefore, rat lung contains a protein with immunological characteristics similar to CYP2J4, and this CYP is reduced after pneumonia. We speculate that CYP2J (but not CYP2B) enzymes and their AA metabolic products (EETs) are involved in the modulation of pulmonary vascular tone in pneumonia in rats.