Effects of nitroglycerin (NG, 3 μg/kg/min, i.v.) on regional myocardial contractile force(MCF) during acute coronary stenosis were studied in 16 open-chest dogs using two strain-gauge arches: one in the normal perfused area and one in the underperfused area supplied by the left circumflex coronary artery (LC-area). MCF in LC-area fell precipitously at coronary perfusion pressures lower than 40 mmHg. The animals were divided into three groups. After complete constriction MCF almost disappeared in Groups I(n=7) and II (n=3), whereas it fell by only a half of control in Group III(n=6). Under normal perfusion, NG increased MCF in Groups I and III, and caused no change in Group II. At pressure 50 mmHg, NG caused an increase in MCF in Groups I and III along with decreases in left ventricular pressure and systemic blood pressure. In Group I, the increase reached to maximal level 2-3 min after NG infusion (130 ± 7 % in LC-area and 118 ± 6 % in normal area) and lasted for at least 10 min. At pressure 40 mmHg, MCF in LC-area decreased in Groups I and II and increased in Group III, although NG changed hemodynamic function to the same levels in the 3 groups. In Group I, 5 min after NG infusion MCF in LC-area decreased to about 70%. At this time, coronary perfusion pressure and coronary blood flow decreased to below 30 mmHg and 37 ± 8 %, respectively. Under these conditions, release of the constriction sufficient for retention of the coronary perfusion pressure at 40 mmHg resulted in significant improvement of MCF in LC-area without a further increase in coronary blood flow. In Group III, 2-3 min after NG infusion MCF in LC-area increased to 112 ± 3 %. The increase was reduced with propranolol; thus at least some participation of β-adrenoceptor is assumed.