Filtration and backfiltration across highly permeable hemodialysis membranes at zero net ultrafiltration rate have been quantified for the F 60, a hemodiafilter with polysulfone hollow fibers (Fresenius, Bad Homburg, FRG), by measuring flow rates and pressure gradients under in vitro and in vivo conditions. Hydrostatic and oncotic pressures as well as the periodic pulsations of blood and dialysate pumps have to be considered. At a blood flow of 200 ml/min and a dialysate flow of 500 ml/min, the amount of backfiltration was calculated to be in vitro (saline) between 30 and 40 ml/min and in vivo in the range of 20 ml/min.