Objective: Experimental studies and opinion articles emphasize that cardiovascular alterations associated with ageing can be improved by the long-term use of fenofibrates. We analyzed the effect of fenofibrate treatment on the acetylcholine-induced relaxation in rat aorta and the participation of nitric oxide (NO) and cyclooxygenase (COX)-derived factors in this effect. Methods: Acetylcholine relaxation in untreated and 6-week fenofibrate-treated Wistar rats was analyzed in the absence and presence of the NO synthase (NOS) inhibitor N G -nitro-L-arginine methyl ester (L-NAME), the specific inducible NO (iNOS) synthase inhibitor 1400W, the nonspecific COX inhibitor indomethacin, the specific COX-2 inhibitor NS-398, the specific thromboxane receptor antagonist SQ-29548, the thromboxane synthesis inhibitor furegrelate, the prostacyclin synthesis inhibitor tranylcypromine, or the 20-HETES synthesis inhibitor formamidine. eNOS, iNOS, COX-1, and COX-2 expression was studied by Western blotting. In addition, production of prostaglandin F 2α (PGF 2α) , thromboxane A 2 (TxA 2 ), prostaglandin E 2 (PGE 2 ), isoprostanes, and prostacyclin (PGI 2 ) was also measured. Results: Fenofibrate treatment reduced acetylcholine relaxation. Indomethacin, NS-398, and tranylcypromine decreased acetylcholine relaxation in untreated rats but enhanced relaxation in treated rats. SQ-29548 increased acetylcholine responses in segments from treated rats but not in segments from untreated rats. L-NAME decreased vasodilator response to acetylcholine in both groups while furegrelate, NS-398, 1400W, and formamidine did not affect acetylcholine responses in either group. eNOS and COX-2 expression was higher in aorta from treated rats while COX-1 and iNOS remained unmodified. Basal and acetylcholine-stimulated NO and PGE 2 release were increased, and that of PGI 2 decreased in treated rats. TxA 2 release was similar, but PGF 2α release was undetectable in both groups. Conclusions: Although it increases NO production through increases in eNOS expression, fenofibrate treatment induces endothelial dysfunction. This effect seems to be mediated by decreased PGI 2 and increased PGE 2 release, and it may help to explain the rise in thromboembolic events observed after long-term fenofibrate treatment in humans.