Ultrafiltration rates (UFR) have been measured for different membranes in a modified dialysis test cell under a range of positive pressures from 50 to 300 Torr. The test cell interior is filled with 200 ml distilled water and water escape rate is measured through one 5·1 cm diameter disc. A linear relationship between UFR and applied pressure exists for cellulose materials and experimental results confirm those of previous workers. A new synthetic membrane presently under clinical evaluation at Strathclyde (Series 10 or “Permane”) shows much higher UFR for any applied pressure compared with cellulose films under the same conditions. A different water transport mechanism is evident from the resulting curves which are obtained by plotting “Permane” UFR against applied pressure. Several methods exist for controlling “Permane” UFR.