In anaesthetized rats a jejunal segment was perfused in situ varying the perfusion rate (0.1, 0.2, 0.5 ml/min) in a randomized order. The intraluminal radius of the segments was small (1.7 mm) or enlarged (3.1 mm) by increasing the intraluminal pressure. The appearance rate of butanol, antipyrine, salicylic acid, D- and L-phenylalanine but not of urea in the venous blood of the jejunal segments was increased up to 35%, when the intraluminal perfusion rate was raised from 0.1 to 0.5 ml/min. Two factors contribute to this effect: the flattening of the concentration gradient down the segment and the reduction of the effective unstirred layer thickness. The length and the intraluminal radius of the perfused segments was not altered, when the perfusion rate was varied. Therefore, a change of the absorbing area did not contribute to the increase of the absorption rate induced by the increase of the perfusion rate. In the series with small intraluminal radius the experimental data corresponded to the theoretical predictions obtained for a laminar intraluminal flow. In the segments with enlarged intraluminal radius the increase of the absorption rate by raising the perfusion rate was less than expected for a laminar flow indicating that the flow might have been turbulent. The enlargement of the intraluminal radius from 1.7 to 3.1 mm increased the absorption rate up to 100%.