Positive end-expiratory pressure (PEEP) ventilation is frequently associated with reduction in cardiac output despite unchanged transmural left ventricular (LV) end-diastolic pressure. These findings have been interpreted to indicate decreased contractility, but could also be explained by altered LV diastolic pressure-volume characteristics. To study this possibility, radiopaque markers were inserted into a plane of the LV in nine dogs. Transmural pressure (LV-pericardial) was synchronized with LV area during ventilation with zero end-expiratory pressure and with 15 cmH2O PEEP. Mean polynomial curves derived from the diastolic pressure-area data demonstrated that PEEP shifted the curves upward so that a given diastolic area was associated with a higher transmural LV pressure (P less than 0.0001). PEEP decreased end-diastolic area and stroke area, both of which were normalized with dextran volume expansion. Restoration of stroke area by normalizing end-diastolic area with volume expansion suggests the initial changes with PEEP were due to a decrease in preload rather than in contractility.