Attempts have been made to demonstrate that the presence of coronary collateral development can preserve and improve the regional contractile force in the ischemic myocardium. Studies were carried out in dogs with developed collaterals and in control dogs. Contractile force was measured with strain-gauge sutured onto the ischemic and the border areas of the myocardium. After ligation of the anterior descending coronary artery, an augmentation of coronary collateral flow via the left circumflex artery was carried out by means of infusion pump up to two and a half fold level. In dogs with developed collaterals, contractile force decreased by 57.6±11.1% (p<0.01) in the ischemic area and 27.2±5.4% (p<0.01) in the border area after coronary ligation. The marked recovery of contractile force up to 63.5±20.4% (p<0.05) of control level in the ischemic are and 70.4±20.4% (p<0.01) in the border are was observed by the augmentation. In control dogs, however, contractile force decreased by 93.2±17.2% (p<0.01) in the ischemic area and 24.6±5.5% (p<0.01) in the border area after the ligation. The augmentation of coronary flow did not recover the depressed contractile force in both areas. These results indicated that the beneficial effect was observed only when the ischemic area was supplied by developed collaterals and closely correlated with the extent of coronary collateral development.