SKF-525A (proadifen) inhibits endothelium-dependent relaxations induced by acetylcholine, arachidonic acid and the calcium ionophore A23187. This suggests that SKF-525A is an inhibitor of endothelium-derived relaxing factor (EDRF) and that EDRF may be a product of arachidonic acid metabolism formed via a cytochrome P-450-dependent pathway or that EDRF release is dependent on cytochrome P-450. We tested this postulate using both isolated rings of rat thoracic aorta and dog mesenteric and femoral artery and the perfusion-superfusion bioassay. Rings of rat thoracic aorta and dog mesenteric and femoral artery with intact endothelium were precontracted with an EC<sub>50</sub> concentration of norepinephrine (0.1 nmol/l) or U46619 (0.05 µmol/l) and the relaxation to acetylcholine (ACh), bradykinin, adenosine triphosphate (ATP) or nitroglycerin (GTN) were obtained before, 30 min after addition of, and 30 min after washout of SKF-525A (50 µmol/l). SKF-525A inhibited ACh-induced endothelium-dependent relaxation of rat aortic rings and endothelium-dependent relaxation of the dog mesenteric and femoral artery produced by ACh and ATP, but did not affect relaxation to bradykinin or GTN. The inhibitory effect on SKF-525A on ACh and ATP-induced relaxation was partially reversed upon its washout from the muscle chamber. Pretreatment of the blood vessels with ibuprofen (1 µmol/l) did not attenuate SKF-525A-mediated inhibition of the relaxations to any agonist. Selective exposure of dog femoral artery (donor) to SKF-525A (50 µmol/l) for 60 min did not affect the relaxation responses of endothelium-rubbed coronary artery (bioassay tissue) to basal EDRF nor to the effluent from donor tissues stimulated with ACh (10–1,000 pmol), ATP (1–100 nmol) or bradykinin (3–100 pmol). The results show that SKF-525A exhibited a reversible inhibition of endothelium-dependent relaxation by a smooth muscle mechanism unrelated to the generation of EDRF from endothelium.