A three-valve pulsatile tubular pump was used in 10 pigs weighing 9.9+/-0.3 kg, connected to a neonatal hemofiltration (HF) circuit to analyze the parameters to determine the ultrafiltration effectiveness for venovenous continuous renal replacement therapy. Forty 30-min periods were studied to analyze the influence of the catheter diameter, purification technique (HF or hemodiafiltration), pump pulsation frequency, percentage time in diastole, percentage time in diastole in which the post pump valve was closed, and percentage time in systole in which the post filter valve was closed, on ultrafiltrate volume and blood flow through the filter. A higher ultrafiltrate flow was achieved with a frequency of 60 bpm than with 30 bpm, a diastolic time of 65% of the total cycle versus 50%, and a blood flow of greater than 20 mL/min versus less than 20 mL/min. The ultrafiltrate flow increases in relation to the blood flow, the frequency of the pump, and the diastolic time.