Regulation of coronary function in diabetic hearts is an important component in preventing ischemic cardiac events, but remains poorly studied. Exercise is recommended in the management of diabetes but its effects on diabetic coronary function are relatively unknown. We investigated coronary artery myogenic tone and endothelial function, essential elements in maintaining vascular fluid dynamics in the myocardium. We hypothesized that exercise reduces pressure induced myogenic constriction of coronary arteries while improving endothelial function db/db mice, a model of type 2 diabetes. We used pressurized mouse coronary arteries isolated from the hearts of control and db/db mice that were either sedentary or exercised for an hour a day on a motorized exercise wheel system (set at 5.2 meters / day, 5 days a week). Exercise caused a ~10% weight loss in db/db mice and decreased whole body oxidative stress as measured by plasma 8-isoprostane levels, but failed to improve hyperglycemia or plasma insulin levels. Exercise also did not alter myogenic regulation of arterial diameter stimulated by increased transmural pressure nor did it alter smooth muscle responses to U-46619(a thromboxane agonist) or sodium nitroprusside (endothelial independent dilator). Moderate levels of exercise restored acetylcholine-simulated, endothelial dependent coronary artery vasodilatation in db/db mice, accompanied by increased expression of manganese superoxide dismutase (MnSOD) and decreased nitrotyrosine levels in the hearts of db/db mice. We conclude that the vascular benefits conferred by moderate levels of exercise were independent of changes in myogenic tone or hyperglycemic status, and primarily involved increased NO bioavailability in the coronary microcirculation.