Quantitative characterization of myocardial properties represents a rapidly emerging area of echocardiographic investigation. Because measurement of the ultrasonic integrated backscatter is theoretically applicable to analysis in vivo with reflected ultrasound, this study was performed to develop and evaluate a suitable method for measurement of quantitative backscatter in vivo. In view of the desirability of characterizing ischemic myocardium noninvasively, the study was performed with animal preparations simulating myocardial ischemia in humans. In one series of open-chest dogs, integrated backscatter among 22 ischemic regions was increased by 200% (P< 0.01) compared to values in control rgions within 1 hour after coronary occlusion and by 400% (−45.1 ± 0.7 dB compared to −50.9 ± 0.4 dB) (P < 0.001) within 6 hours. In a second series of open-chest dogs, ischemia was quantified with141Ce microspheres, and mean integrated backscatter was found to increase (280% of control) {P < 0.01) in regions with flow less than 20% of control 2 hours following coronary occlusion. Additional studies -with perfused hearts revealed two determinants of the increased ultrasonic backscatter observed: (1) an increase in cardiac fluid content reflected by the wet-to-dry weight ratio, and(2) the contributions of formed elements in whole blood. The results indicate that ultrasonic integrated backscatter distinguishes severely ischemic from nonischernic myocardium in vivo in open-chest animnlK. Because it was possible to obtain these results in the reflection mode, potential extension of the approach to clinical applications is promising.