The present study addresses the effect of a sustained change in pressure on microvascular permeability assessed by hydraulic conductivity (L_p) measurements from microvessels of the rat mesentery. Using a micro-perfusion technique, transvascular filtration (normalized to surface area; J_v/S) and L_p were measured in small arterioles (baseline L_p = 0.26 x 10^-7 cm s^-1 cm H2O^-1) and venules (baseline L_p = 2.88 x 10^-7 cm s^-1 cm H2O^-1). The main finding of this study is that step increases in microvascular pressure led to time-dependent alterations of L_p. Immediately after a two-fold step increase in pressure, J_v/S increased in proportion to the pressure change. This observation is consistent with Starling's Law that predicts filtration proportional to the overall pressure gradient when L_p is constant. However, when J_v/S measurements continued for 60-90 minutes past the step in pressure, there was an initial decrease in J_v/S for 30 min ("sealing effect") followed by a substantial increase in J_v/S out to 90 min. The sustained increase in J_v/S suggests an increase in L_p of 36 ± 7% for small arterioles and 42 ± 5% for small venules, P<0.05 for both. In addition, the increase in L_p in response to an increase in pressure was attenuated significantly by nitric oxide synthase (NOS) inhibition. These results indicate that a pressure-induced mechanical stimulus (possibly J_v) activates a nitric oxide-dependent biochemical response that leads to an increase in hydraulic conductivity.