To evaluate the functional characteristics of continuous veno-venous hemofiltration in a pediatric size animal model. Prospective trial. Animal laboratory at a large university-affiliated medical center. Four-week old lambs (weight 12.2 +/- 1.3 kg). Veno-venous hemofiltration was performed in anesthetized lambs (n = 5, 12.2 +/- 1.3 kg) using a standard pediatric hemofilter and pumped blood and ultrafiltrate. We compared postdilution, predilution, and hemofiltration with counterflow dialysis. At net ultrafiltrate flows of 200, 400, and 600 mL/hr, we measured system pressures and urea clearance. Stable blood flow could reproducibly be achieved up to 140 mL/min (10 mL/kg/min); at higher flow demand, tubing collapse occurred. At blood flow rates of 5 to 10 mL/kg/min, ultrafiltrate flow of 1 mL/kg/min would create negative filter compartment pressure but consistently less negative than -500 mm Hg. During postdilution, predilution, and counterflow dialysis, urea clearance was virtually equal to ultrafiltrate flow. There was no increase in urea clearance when adding predilution or dialysis to basic postdilution. Veno-venous hemofiltration, using small filters and circuits in a pediatric size animal, can achieve stable blood flow up to 10 mL/kg/min. At this flow, ultrafiltrate flow of 1 mL/kg/min can produce a urea clearance of 1 mL/kg/min while keeping filter compartment pressure above maximal recommended negative pressures. Addition of dialysis in this nonuremic model did not increase urea clearance.