The aims of this study were to investigate myogenic tone in cannulated coronary arteries from SHR (spontaneously hypertensive rats) and WKY (Wistar-Kyoto rats) at 5 and 20 weeks of age under pressurised no-flow conditions. Pressure-diameter relationships (20-200 mm Hg) were determined in the presence (active) and absence (passive) of calcium and myogenic tone at each pressure was calculated. Active, but not passive diameter-pressure relationships were significantly different between strains at both ages. Active diameters were similar between strains at pressures up to 140 mm Hg at 5 and 20 weeks. At higher pressures SHR arteries generated more tone. Maximum myogenic tone was similar between strains in spite of increased wall thickness in the SHR at both ages. Endothelium denudation increased myogenic tone to a greater extent in the SHR at both ages. Active diameters of endothelium-denuded SHR arteries were smaller than those of WKY in the range 20-120 mm Hg at 5 week and 20-80 and 160-180 mm Hg at 20 weeks. These results demonstrate that during both the developmental and established phases of hypertension, myogenic tone is not enhanced in intact SHR coronary arteries, except at the highest pressures. Following endothelium removal SHR arteries generated greater myogenic tone also at lower pressures. These data demonstrate a greater endothelium-mediated antagonism of myogenic tone in SHR coronary arteries.