The functional characteristics of the capillary membranes of rat kidneys were analysed in a computer-based model utilizing previous data on the hydraulic conductivity of the membranes and transport by diffusion, hydraulic flow and ion migration of: inulin, myoglobin (negative, neutral and positive), horseradish peroxidase (negative and neutral), lactate dehydrogenase (negative, neutral and positive) and albumin. The results showed that in the peritubular capillaries the main fluid reabsorption (26.8 x 10(-8) ml nephron-1 100 g body wt-1) occurs through a set of 20 A pores with a total pore area over pore length of 30 cm x (nephron 100 g body wt)-1, whereas the plasma proteins enter the renal interstitium through a few 140 A large pores with a total pore area over pore length of 7.2 x 10(-4) cm and a fluid reabsorption of 1.2 x 10(-10) ml nephron-1 100 g body wt-1. The intramembranous concentration of negative fixed charges in the large pore system was estimated at 20 mM (range 15-22 mM). Since this was accompanied by a potential difference of 0.3 mV, a net driving electro-osmotic force, favouring reabsorption, of 4 mmHg developed. The charges and the electro-osmotic force was found to be essential for protein transport, since if the membrane was uncharged, the transport and interstitial concentration of e.g. albumin was twice as high. The glomerular capillaries seem to have a more homogeneous structure, that is in essence a one-pore system; the pore radius was 44 A, the total pore area over pore length 2.8 cm and the concentration of negative fixed charges 40 mM (range 37-43 mM).