We studied the hemodynamic effects of acute alterations in PaCO2 in ten ventilator-dependent children after open heart surgery. Despite end-tidal CO2 monitoring, five children inadvertently developed PaCO2 greater than 50 torr during the study. Increasing and decreasing PaCO2 in these children resulted in significant parallel changes in cardiac index (CI), oxygen delivery, physiologic shunt, mean pulmonary pressure, and right ventricular stroke work index, and inverse changes in systemic vascular resistance index (p less than .01). No significant hemodynamic changes were observed with alterations in PaCO2 in children whose PaCO2 remained less than 50 torr. These findings are partly related to significantly larger reductions in PaCO2 which occurred in the hypercarbic children compared to the others (25 vs. 12 torr; p less than .001). Heart rate, mean arterial BP, and venous filling pressures did not change significantly in either group. We conclude that mean alterations in PaCO2 less than or equal to 12 torr (range 28 to 50) have no significant effect on CI in children after open heart surgery. However, moderate hypercarbia and its correction to a mean PaCO2 of 30 torr are associated with significant effects on central hemodynamics. Moreover, no significant changes in the commonly monitored physiologic variables were observed despite large variations in PaCO2.