Experimental myocardial ischemia produced in dogs by proximal left anterior descending coronary artery ligation is accompanied by relatively rapid (1 h) increases in the number of (-) [3H]dihydroalprenolol binding sites without changing their dissociation constants in ischemic left ventricular tissue. The changes, persist for at least 8 h and are accompanied by marked decreases in myocardial tissue ischemic region norepinephrine content. In contrast, in the same canine model 1 h of proximal left anterior descending coronary artery ligation did not result in a significant change in the number of [3H]quinuclidynl benzilate binding sites of their dissociation constants. However, the number of [3H]quinuclidynl benzilate binding sites (muscarinic cholinergic receptors) are 50--70% greater than (-) [3H]dihydroalprenolol binding sites (beta adrenergic receptors) in canine left ventricular tissue. Thus, the data suggest that proximal left anterior descending coronary artery occlusion for 1 h significantly increases the number of beta adrenergic receptors in ischemic left ventricular tissue without changing the number of muscarinic cholinergic receptors. Whether the ischemia-produced increase in cardiac beta-receptor content is causally related to increased cyclic AMP levels that develop in ischemic tissue and/or an etiologic factor in arrhythmias originating from ischemic myocardial tissue will have to be determined in additional studies.