Background: This study was conducted to determine the effects of chronic combined pulmonary stenosis and pulmonary insufficiency (PSPI) on right (RV) and left (LV) ventricular function in young growing swine. Methods and Results: Six pigs with combined PSPI were studied and data were compared to previously published data of animals with isolated PI and controls. Indices of systolic function (stroke volume, ejection fraction and cardiac functional reserve), myocardial contractility (slope of the endsystolic pressure-volume and dP/dtenddiastolic- volume relationship [E_max and M_dp/dt]) and diastolic compliance [kappa] were assessed within two days of intervention, and three months later. Magnetic resonance imaging (MRI) was used to quantify pulmonary insufficiency and ventricular volumes. The conductance catheter was used to obtain indices of the cardiac functional reserve, diastolic compliance and myocardial contractility from pressure-volume relations acquired at rest and under dobutamine infusion. In the PSPI group the pulmonary regurgitant fraction was 34.3±5.8%, the pressure gradient across the site of pulmonary stenosis was 20.9 ± 20 mmHg and the average RV peak systolic pressure was 70% systemic at 12 week follow-up. Biventricular resting cardiac outputs and cardiac functional reserves were significantly limited (p<0.05), LV diastolic compliance significantly decreased (p<0.05) but RV myocardial contractility significantly enhanced (p<0.05) when compared to control animals at three months follow-up. Conclusions: In the young developing heart chronic combined pulmonary stenosis and insufficiency impairs biventricular systolic pump function and diastolic compliance but preserves RV myocardial contractility.