Since the use of nitroglycerin (NTG) in cases of acute myocardial infarction is controversial, the effects of this drug were studied using experimental animals. NTG was given to open chest dogs during induced acute ischemia and the motion and the wall thickness of left ventricle were examined using the method of Gaasch and Bernard. Mongrel dogs weighing 10 to 25kg (average 15kg) were anesthesized with urethane, alpha-chloralose and morphine. A mini-transducer was immobilized on the surface of left ventricular wall and the motion and thickness of left ventricular wall were measured by Ultrasound. In some dogs, an electromagnetic flowmeter probe was attached to the left anterior descending coronary artery and the coronary blood flow and aortic pressure were measured. One of diagonal branches was then ligated, acutely. After a steady state had been reached, 30 gamma/kg or 100 gamma/kg of NTG was administered intravenously and changes in the wall thickness were measured serially. After this ligation, the wall thickness decreased rapidly and reached a plateau 90-120 min after. Following administration of 30 gamma/kg and 100 gamma/kg of NTG, both end-systolic wall thickness and maximal rate of wall thickness during systole were restored, the former 12.7 +/- 2.2 to 13.7 +/- 2.0 mm and 12.5 +/- 2.5 to 13.9 +/- 2.5 mm (p less than 0.05 in both), the latter from 37.0 +/- 16.7 to 55.0 +/- 26.5 mm/sec and 41.0 +/- 27.8 to 56.0 +/- 33.7 mm/sec (p less than 0.001 in both). Concomitant with these effects were a transient increase in coronary blood flow and a decrease in aortic pressure. These observations indicate that abnormal wall motion and performance of the left ventricle are improved with NTG administration, thus this drug should be clinically effective for treating patients with acute myocardial infarction.