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Summary: To evaluate the effects of left ventricular contractility on the changes of average image intensity (AII) of the myocardial integrated backscatter (IB) and cyclic variation in IB (CVIB), 7 adult mongrel dogs were studied. The magnitude of AII and CVIB were measured from myocardial IB carves before and after dobutamine or propranolol infusion. Dobutamine or propranolol did not affect the magnitude of AII (13.8±0. 7 vs 14.7±0. 5, P＞0. 05 or 14.3±0.5 vs 14.2±0. 4, P＞0. 05). However, dobutamine produced a significant increase in the magnitude of CVIB (6.8±0.3 vs 9.5 ± 0. 6, P＜0. 001) and propranolol induced significant decrease in the magnitude of CVIB (7.1±0. 2 vs 5.2±0. 3, P＜0. 001). The changes of the magnitude of AII and CVIB in the myocardium have been demonstrated to reflect different myocardial physiological and pathological changes respectively. The alteration of contractility did not affect the magnitude of AII but induced significant change in CVIB. The increase of left ventricular contractility resulted in a significant rise of the magnitude of CVIB and the decrease of left ventricular contractility resulted in a significant fall of the magnitude of CVIB.