1. The effect of adenosine on left ventricular contractility and developed tension was studied in the anaesthetized intact dog. Normal and propranolol-treated animals were used. Adenosine was infused through a catheter into one of the two main branches of the left coronary artery, usually the circumflex branch. The rate of infusion was 150 x 10(-9)M/min. The infusion was maintained during 50 min.2. In both series of animals, no change was observed in the heart rate, aortic pressure, left ventricular end-diastolic pressure, time from onset of left ventricular contraction to peak dP/dt, peak dP/dt and left ventricular tension-time index. It is concluded that a regional increase in adenosine concentration in the left ventricular wall has no inotropic effect when the adrenergic mechanisms are normal or depressed.3. The myocardial blood flow response to adenosine was determined at the 10th, 30th and 50th min of the infusion by the radioactive inert gas method. At the 10th min of the infusion, the myocardial blood flow averaged three times the control value in both series of dogs. Thereafter, the flow response remained stable in the normal dogs but declined at the 50th min of the infusion in the propranolol-treated animals. It is suggested that autoregulation of the coronary circulation in response to overperfusion of the myocardium at constant cardiac work may be enhanced at the lower myocardial oxygen requirements of the propranolol-treated dogs while, in the normal animals, it was insufficient to overcome the potent coronary dilator action of adenosine.