Members of the P450 4A subfamily are key enzymes in the synthesis and degradation of metabolites of arachidonic acid, which are of physiological importance in the brain. In the rat, four members of this subfamily, 4A1, 4A2, 4A3, and 4A8, have been described. In this study, the expression of members of the 4A subfamily in the rat brain has been examined by PCR amplification, by western and northern blotting, and by protein N-terminal sequencing. With PCR all four members of the subfamily were detectable in the liver and kidney. P450 4A1 was found exclusively in the liver and kidney, whereas P450 4A2 was detectable in all the tissues tested, including the lung, seminal vesicles, prostate, cerebral cortex, hypothalamic preoptic area, cerebellum, and brainstem. The tissue distribution of P450 4A3 was similar to that of 4A2 except that it was not detectable in seminal vesicles. A P450 4A8-specific fragment was amplified from the kidney, liver, and prostate and weakly from the cerebral cortex but not from other brain regions. Despite the evidence of their presence by PCR, no members of the 4A family were detectable on northern blots with mRNA from the brain. On western blots a P450 4A-specific antiserum recognized a band in P450 fractions prepared from the brain. The intensity of the signal with 30 pmol of P450 from the brain was similar to that with 10 pmol of liver microsomal P450. The brain P450 was extracted from 1 g of brain, whereas the 10 pmol of liver P450 is the equivalent of 1 mg of liver.(ABSTRACT TRUNCATED AT 250 WORDS)