The expression of P-450 4A isoforms responsible for the formation of 20-hydroxyeicosatetraenoic acid (20-HETE) was examined using the reverse transcription and polymerase chain reaction in various nephron segments and preglomerular arterioles microdissected from the kidneys of Sprague-Dawley rats. Expression of cytochrome P-450 4A1, 4A2, 4A3, and 4A8 mRNA could be detected in RNA extracted from the whole kidney. The expression of P-450 4A1, 4A3, and 4A8 mRNA was similar in the kidney of male and female rats, whereas the expression of 4A2 mRNA was fourfold greater in the kidney of male vs. female rats. At the single-nephron level, P-450 4A1 mRNA could not be detected in either preglomerular arterioles or any nephron segments. P-450 4A2 mRNA was readily detected in preglomerular arterioles, glomeruli, proximal convoluted tubule (PCT), proximal straight tubule (PST), medullary thick ascending limb (MTAL), cortical thick ascending limb (CTAL), cortical collecting duct (CCD), outer medullary collecting duct (OMCD), and inner medullary collecting duct (IMCD). P-450 4A3 mRNA was also detected in every nephron segment, but the expression of this isoform was barely detectable in preglomerular arterioles. The expression of P-450 4A8 mRNA was detected in the glomerulus, PCT, PST, CTAL, and CCD. It was not detectable in preglomerular arterioles, MTAL, OMCD, or IMCD. Immunoblot analysis using a P-450 4A antibody exhibited a strong signal for P-450 4A protein in the proximal tubule. Smaller signals were also observed in glomerulus, MTAL, and preglomerular arterioles, but no signal could be detected in the IMCD. A similar pattern of P-450 4A protein expression was seen in kidney sections immunostained with this antibody. These results indicate that the expression of P-450 4A isoforms in the kidney of rats is sex dependent and that different P-450 4A isoforms are expressed throughout various nephron segments and the renal vasculature of rats.