BACKGROUND To determine how the tissues that surround the heart affect diastolic and systolic function during the perinatal period, we studied the pressure-diameter relation of the left ventricle in partially delivered fetal lambs. RESULTS We anesthetized (1.5-2.0% halothane, balance O2) and ventilated six pregnant ewes (142-144 days of gestation) and then partially delivered each lamb by cesarean section. Each lamb was instrumented to record left ventricular anteroposterior diameters (endocardial ultrasonic transducers), pericardial pressure (liquid-containing balloon), and left ventricular pressure (transducer-tipped catheter). Left ventricular pressure-diameter relations were recorded under three conditions: initially, with a closed chest and closed pericardium (before ventilation); second, after interruption of the umbilical circulation and 1 hour of ventilation; and finally, when the lungs and the pericardium were retracted from the heart. Pericardial pressure (recorded at a common diameter, i.e., the maximal end-diastolic diameter recorded before ventilation) decreased by 48% after 1 hour of ventilation (p < 0.05). After ventilation, left ventricular anteroposterior diameters were 4-5% greater (p < 0.05) at each end-diastolic pressure compared (12.5, 15.0, 17.5, and 20 mm Hg). Thus, ventilation appeared to increase left ventricular diastolic compliance. Contractility also appeared to increase after ventilation when evaluated using ventricular stroke work as a function of end-diastolic pressure as preload. When we used a more appropriate measure of preload (i.e., transmural end-diastolic pressure), ventilation did not change left ventricular diastolic compliance or contractility. Thus, left ventricular systolic function increased because of an increase in preload. CONCLUSIONS The tissues surrounding the fetal heart significantly augment pericardial pressure and limit left ventricular preload. The initiation of ventilation reduces pericardial pressure, increases left ventricular preload, and increases left ventricular systolic function. At birth, a decrease in pericardial pressure and the resulting increase in preload may help increase left ventricular output through the Frank-Starling mechanism.