Abstract: The variation of ultrafiltration flow rate (QF) at high transmembrane pressure with inlet wall shear rate (γW) was found to be proportional to γW, with n ranging from 0.45 to 0.55, when γW increases up to 4,500 s‐1. To test whether long filters operated at high shear rates were more efficient than shorter ones for the same inlet blood flow, we made experiments with 10‐and 20‐cm filters with same number of fibers (550) at various shear rates from 700 to 3,000 s‐1. The filtration rates provided by the 20‐cm filter were found to be 5‐15% larger than those provided by two 10‐cm filters arranged in parallel and 10‐20% smaller than those provided by two 10‐cm filters arranged in a series. The explanation lies in the rapid decay of QF with distance from the inlet due to the developing concentration boundary layer. When pulsations are imposed on the inlet blood flow, the filtration rate was seen to increase by 10‐20%, and the effect of plugging was seen to decrease in small‐area hemofilters.