The generation of PGs from arachidonic acid is mediated by cyclooxygenase (COX), which consists of a constitutive (COX-1) and an inducible (COX-2) isoform. The present study evaluated the relative expression and immunoreactive levels of COX-1 and COX-2, by means of RT-PCR, Western blot analysis, and immunohistochemistry, in the renal cortex and medulla of rats with congestive heart failure (CHF), induced by the placement of an aortocaval fistula. In addition, we examined the effects of a COX-1 inhibitor (piroxicam), COX-2 inhibitor (nimesulide), and nonselective COX inhibitor (indomethacin) at a dose of 5 mg/kg, on intrarenal blood flow by laser Doppler flowmetry. COX-1 and COX-2 mRNAs were abundantly expressed in the renal medulla of control and CHF rats and only minimally in the cortex. Moreover, both RT-PCR (32-36 cycles) and Western blot techniques revealed upregulation of medullary COX-2, but not of COX-1, in rats with advanced heart failure. In line with these findings, all three tested COX inhibitors provoked significant and sustained decreases (Delta approximately -20%) in medullary blood flow (MBF), which were similar in magnitude and duration in control animals. However, in CHF rats, indomethacin produced a greater reduction in MBF than that obtained with either piroxicam or nimesulide. Taken together, these results indicate that 1) both COX-1 and COX-2 are predominantly expressed in the renal medulla and 2) experimental CHF is associated with selective overexpression of COX-2. The latter may represent a mechanism aimed at defending MBF in the face of a decrease in renal perfusion pressure during the development of CHF.