Transmission electron microscopic techniques were used to carry out a quantitative analysis of the density of fenestration in the inner medullary vasa recta of the rat kidney. Measurements were made at 200 µm intervals from the tip to the base of the papilla (1800 µm from the tip). Fenestral diaphragms were estimated to be 65·4 ± 0·78 nm in diameter (mean ± S.E.M.), and were arranged in plaques with a mean interfenestral distance of 114·8 ± 2·6 nm. Near the tip of the papilla there was no correlation between vessel size and degree of fenestration; density of fenestration, however, began to decrease about 1400 µm from the tip. The ratio of fenestrated to non-fenestrated profiles of vasa recta was found to be linear with respect to distance from the tip (r = 0·991), with values ranging from about 40 : 1 near the tip to 2 : 1 near the base of the papilla. We have estimated the proportion of the total surface area of a fenestrated vasa recta occupied by fenestral diaphragms to be 0·057 at 1000 µm from the tip. The total potential conductance (K) of a 200 µm segment of fenestrated vessel at 1000 µm from the tip was calculated to be 0·319 µm3 s-1 cmH2O-1, giving a hydraulic conductivity (Lp) of 0·030 µm s-1 cmH2O-1. We have also examined the reverse question of the conductance of a single fenestra if all the fluid flux across the vessel wall occurred through the fenestrae and none via the intercellular clefts or water channels; single fenestral conductance was estimated to be 1·94 × 10-3 µm3 s-1 cmH2O-1.