The aim of the study was to evaluate whether there was an imbalance between endothelin-1 (ET-1) and nitric oxide (NOx) release and diffuse atherosclerotic changes existed in patients with slow coronary flow (SCF). Baseline and post-atrial pacing coronary sinus ET-1 and NOx levels were measured in 19 patients with SCF (11 female, 56±9 years) and in 14 control subjects (nine female, 54±7 years). All patients underwent subsequent intravascular ultrasound (IVUS) investigation at the same setting with right atrial pacing. Baseline arterial (12.4±9.9 vs. 6.3±5.1 pg/ml, P<0.005) and coronary sinus (12.2±11.1 vs. 6.4±6.9 pg/ml, P<0.005) ET-1 plasma levels were higher in patients than in controls. After atrial pacing, concentration of ET-1 level from coronary sinus (24.7±14.6) significantly increased as compared to baseline (12.4±9.9, P<0.0001) and control levels (5.3±6.3, P<0.0001). Additionally, coronary sinus ET-1 level increased significantly with atrial pacing compared to femoral artery ET-1 level (16.3±8.5, P<0.005) in patients with SCF. After atrial pacing, the femoral artery ET-1 level also increased in patients compared to control level (P<0.0001). No significant differences in arterial and coronary sinus NOx plasma levels were found between the two groups, both at baseline and after pacing. Upon IVUS investigation, the common finding was longitudinally extended massive calcification throughout the epicardial arteries in patients with SCF. Mean intimal thickness was 0.59±0.18 mm. The data of this study suggest that increased ET-1 levels and insufficient NOx response, as well as the pathological data of IVUS may be associated with coronary microvascular dysfunction and may be the manifestation of early diffuse epicardial atherosclerosis in these patients with SCF.