The hydraulic conductivity of the peritubular capillary membrane was calculated from 1) single nephron fluid reabsorption and 2) net driving force, i.e. from hydrostatic and colloid osmotic pressures in renal interstitium and peritubular capillary blood, as determined by a micropuncture technique and with use of a computer-based model. Under control conditions the net driving force was estimated to be 15.4 mmHg and the hydraulic conductivity 1.04 nl/(min . mmHg) per 100 g rat. During extracellular volume expansion with 0.15 M saline, 4% and 10% of body weight, the net driving force decreased to 12.5 mmHg and 6.4 mmHg, respectively, whereas the conductivity increased to 1.85 and 3.14 nl/(min . mmHg) per 100 g rat. The reduction in net driving force was compensated by an increased hydraulic conductivity. In the glomeruli the net driving force for filtration increased from 14.2 mmHg under control conditions to 21.2 mmHg and 25.3 mmHg during saline expansion 4% and 10%, whereas the corresponding hydraulic conductivity de increased from 1.13 nl/(min . mmHg) per 100 g rat to 1.03 and 0.80 nl/(min . mmHg) per 100 g rat during the two expansions. During saline expansion the water permeability of the glomerular capillaries is decreased while that of the peritubular capillaries is increased. These changes in the water permeability will lead to retarded excretion of the excess fluid.