Abstract Corticosteroids frequently are used during cardiopulmonary bypass (CPB) to enhance total body perfusion and myocardial preservation. The mechanisms by which steroids might provide protection to the myocardium have not been clearly defined, however. Therefore this study was performed to measure the effects of methylprednisolone (M-P) on coronary flow and distribution, and on myocardial metabolism and contractility. Twenty-three dogs underwent 1 hour of total CPB, 80 cc/kg/minute at normothermia with beating hearts. Alternate animals received M-P, 30 mg/kg. Myocardial blood flow (microspheres technique), myocardial tissue lactate and adenosine triphosphate, lactate extraction, coronary sinus flow (CSF), and coronary vascular resistance (CVR) were measured before, during, and 60 minutes after bypass. LV dp/dt and cardiac output (CO) were measured before and after bypass. Total coronary flow was significantly higher in the M-P group after 10 and 30 minutes of bypass (93 vs 56 ml/100 gm/minute, p < 0.05, and 96 vs 71 ml/100 gm/minute, p < 0.05). Right ventricular flow was higher in the M-P group at 10 and 30 minutes of bypass (98 vs 66 ml/100 gm/minute, p < 0.05, and 90 vs 78 ml/100 gm/minute). Left ventricular flow was higher in the M-P group at 10 minutes of bypass (79 vs 52 ml/100 gm/minute, p < 0.08). Septal flow also was higher in the treated group at 10 minutes of bypass (64 vs 49 ml/100 gm/minute) and at 30 minutes of bypass (92 vs 67 ml/100 gm/minute, p < 0.05). CVR after 10 minutes of bypass was lower in the steroid group (88 mm Hg/ml/100 gm/minute vs 1.39 in the control group, p < 0.03). It is concluded that M-P increases coronary blood flow and decreases CVR in the empty beating heart during normothermic CPB without altering myocardial metabolism or contractility.