To investigate the properties of cromakalim-opened K⁺ channels in aorta of spontaneously hypertensive rats (SHR), the effect of cromakalim on tension was compared in endothelium-rubbed aortic rings from SHR and normotensive Wistar-Kyoto rats (WKY). 1. Cromakalim relaxed the aortic ring contracted by 10^-7 M norepinephrine (NE) dose-dependently, and this relaxant response to cromakalim was blocked by 10^-5 M glybenclamide. 2. Cromakalim also relaxed the contraction induced by high K⁺-solution or 10 mM tetraethylammonium dose-dependently. However, the relaxant response to cromakalim was decreased by raising the K⁺ concentration. 3. SHR aorta exhibited myogenic tone in resting state which was inhibited by cromakalim, verapamil or Ca²⁺-free PSS. Whereas, WKY aorta did not exhibit any myogenic tone in resting state. 4. When aortic rings from both strains were contracted by 20 mM K⁺ or NE, relaxant responses to low concentration of cromakalim (below 10^-7 M) were not different between WKY and SHR, but maximum relaxant response to cromakalim (above 3 X 10^-7 M) was greater in SHR than in WKY. 5. When the relaxant response to cromakalim was expressed as percent of maximum relaxation induced by Ca²⁺-free PSS, relaxant response to cromakalim in 20 mM K⁺-induced contraction was not different between WKY and SHR. From the above result, it is suggested that relaxant responses to cromakalim are greater in SHR than WKY, and this may be due to the myogenic tone of aortic rings from SHR.