These experiments report the first direct measurement of microvessel permeability coefficients after exposure to vascular endothelial growth factor (VEGF). The Landis technique was extended to enable measurement of the resistance of the microvessel wall to water flow, hydraulic conductivity (Lp), on the same microvessel in the frog mesentery during the initial exposure to VEGF (acute) and 24 and 72 h after initial exposure (chronic). Control measurements of Lp showed no change either acutely or chronically. Exposure to 1 nM VEGF rapidly and transiently increased microvessel Lp within 30 s (to 7.8-fold greater than baseline values) and returned to control within 2 min. The baseline Lp was fivefold greater after 24 h than the initial baseline as a result of VEGF perfusion and returned to its original value after 72 h. These experiments confirm the hypothesis that VEGF acts both acutely (over a period of a few minutes) and chronically (over a few hours) to increase microvascular permeability.