Positive end-expiratory pressure (PEEP) is used liberally whenever a ventilated patient shows signs of increased pulmonary venous shunting. Clinicians using PEEP to improve blood oxygenation may face the cardiovascular side effects which limit utilization of the desired respiratory effects of PEEP. We measured the pressure flow characteristics of the cardiovascular system and the coronary arterial system as a function of PEEP, using closed-chest surgically instrumented dogs, in order to assess its effects on myocardial blood flow with respect to the left ventricular energy demands. The aortic left ventricular blood pressure as well as the aortic blood flow decreased with increasing PEEP values. The coronary blood flow decreased by 5% for PEEP values of 4 cm H<sub>2</sub>O, and by 25% for 14 cm H<sub>2</sub>O of PEEP. PEEP values under 10 cm H<sub>2</sub>O reduced the left ventricular end-diastolic pressure (LVEDP), while higher PEEP values caused an increase in LVEDP. The relation between the alterations of coronary and aortic blood flows changed with PEEP values. Low PEEP values (less than 10 cm H<sub>2</sub>O) had a tendency for higher relative reduction of aortic blood flow, whereas higher PEEP values (higher than 10 cm H<sub>2</sub>O) reduced the coronary blood flow more than the reduction occurring in the aortic blood flow. Our results suggest that low PEEP values may have beneficial effects on the relation between aortic blood flow and coronary blood flow, therefore low PEEP application may minimize hypoxic myocardial alterations. Further studies that will measure left ventricular workload or another metabolic index for estimating myocardial perfusion relative to its metabolic demand are essential before clinical conclusions can be drawn from our results.