The whole blood filtration rate of hemofilters were measured in in vitro circuits simulating the pressure and flow conditions of arteriovenous filtration. For all devices tested, filtration rate was found to be independent of blood velocity and wall shear rate but to increase linearly with applied transmembrane pressure according to the following relationship: QF = K(TMPH -Δπ), where QF is device filtration rate; K is device water permeability, determined post exposure to blood; TMPH is applied transmembrane pressure; △π is colloid osmotic pressure in plasma. Such behavior suggests that transport in AV filtration is membrane limited; this is in contrast to earlier models which were based upon boundary layer concentration polarization or unfavorable osmotic gradients.