There are no studies analyzing the initial hemodynamic impact of continuous renal replacement therapy (CRRT) in children. We have performed a prospective observational study in 34 immature Maryland pigs to analyze the initial hemodynamic changes during venovenous CRRT. The heart rate, blood pressure, central venous pressure (CVP), pulmonary arterial occlusion pressure (PAOP), pulmonary capillary wedge pressure, temperature, and cardiac output (CO), simultaneously by pulmonary arterial thermodilution and femoral arterial thermodilution, were measured at 30-min intervals during 2 h. Venovenous CRRT induced an initial significant diminution of volemic hemodynamic parameters (intrathoracic blood volume, global end-diastolic volume, stroke volume index, PAOP, and CVP). Simultaneously, a significant increase in systemic vascular resistance index and left ventricular contractility, and a decrease in CO, was observed. We conclude that CRRT in a pediatric animal model induces initial hypovolemia, and a systemic cardiovascular response with vasoconstriction and increase in ventricular contractility.