The effects of the pericardium on myocardial diastolic ventricular interaction during early development were determined in vitro using hearts excised from eight preterm (109 ± 0.106 SE d gestation; term = 147 d) and eight newborn (2.3 ± 0.30 SE d postnatal age) lambs. The lamb hearts were excised with the pericardium intact and immersed in a cold cardioplegic solution. Subsequently, very compliant balloon catheters were inserted into the right and left ventricles retrograde through the pulmonary artery and aorta respectively. Before and after the removal of the pericardium, the changes in left ventricular pressure or volume decreased by increasing right ventricular pressure or volume were measured. Computerized analysis of the pressure and volume recordings yielded pressure and volume transfer functions that quantified ventricular interaction. The pressure transfer functions (left ventricular pressure/right ventricular pressure) both with and without the pericardium intact were 0.297 ± 0.010 SE and 0.140 ± 0.005 SE, respectively, for the preterm and 0.650 ± 0.033 SE and 0.301 ± 0.031 SE for the newborns. The volume transfer functions (left ventricular volume/right ventricular volume) with and without the pericardium were 0.320 ± 0.04 SE and 0.150 ± 0.024 SE, respectively, for the preterm and 0.514 ± 0.052 SE and 0.233 ± 0.027 SE for the newborns. These data demonstrate that 1) the pericardium increases ventricular interaction in both preterm and newborn lambs and 2) the relative percentage increase is similar for both age groups and not age dependent. Furthermore, these findings suggest that the increase in ventricular interaction due to the presence of the pericardium is mediated by the relative compliances of the free walls, septum, and pericardium.