Platelet-derived growth factor (PDGF) has been shown to act chronically on blood vessels to regulate not only proliferation but also vascular tone. These effects may be at least partly due to the chronic effect of PDGF on vascular endothelium. To evaluate this possibility, we examined the effects of PDGF on the endothelium-dependent relaxation (EDR) and total RNA for endothelial nitric oxide (NO) synthase (eNOS) using an organ culture system. In rabbit mesenteric arteries cultured in a serum-free medium for 1 wk, amplitude of the substance P-induced EDR did not change, whereas dependency of the EDR on NO (approximately 60.0% vs. 18.9%) and the total amounts of recoverable eNOS mRNA estimated by RT-PCR were increased compared with those in freshly isolated arteries. Culture with PDGF for 1 wk decreased the relaxant effect of substance P and ionomycin (P < 0.01 compared with the arteries without PDGF), NO production estimated by bioassay (P < 0.01), and eNOS mRNA level, whereas the sodium nitroprusside-induced relaxation did not change. These results suggest that PDGF has a chronic effect on vascular endothelium to decrease eNOS mRNA and NO production and to impair NO-dependent EDR.