Endurance exercise training increases basal active tone in coronary arteries and enhances myogenic tone in coronary arterioles of control animals. Paradoxically, exercise training has also been shown to augment nitric oxide production and nitric oxide-mediated relaxation in coronary arterioles. The purpose of the present study was to examine the effect of exercise training on basal active tone of arterioles (~150 µm ID) isolated from the collateral-dependent region of hearts exposed to chronic coronary occlusion. Ameroid occluders were surgically placed around the proximal left circumflex coronary artery of miniature swine. Arterioles were isolated from both the collateral-dependent and nonoccluded myocardial regions of sedentary (pen-confined) and exercise trained (treadmill run; 14 wks) pigs. Coronary tone was studied in isolated arterioles using microvessel myographs and standard isometric techniques. Exposure to nominally Ca²⁺-free external solution reduced resting tension in all arterioles; decreases were most profound (P<0.05) in arterioles from the collateral-dependent region of exercise trained animals. Furthermore, nitric oxide synthase (NOS) inhibition (L-NAME; 100 µM) unmasked markedly increased nitric oxide-sensitive tone in arterioles from the collateral-dependent region of exercise trained swine. Blockade of K⁺ channels revealed significantly enhanced K⁺ channel contribution to basal tone in collateral-dependent arterioles of exercise trained pigs. Protein content of eNOS and phosphorylated-eNOS (pS1179) determined by immunoblot was elevated in arterioles from exercise trained animals with the greatest effect in collateral-dependent vasculature. Taken together, we demonstrate the interaction of opposing exercise training-enhanced arteriolar basal active tone, nitric oxide production, and K⁺ channel activity in chronic coronary occlusion, potentially enhancing the capacity to regulate blood flow to collateral-dependent myocardium.