The red cell microperfusion-micro-occlusion technique has been used to measure fluid filtration and reabsorption (Jv/A) at known microvascular pressures (Pc) in single mesenteric venules of anaesthetized rats. The relation between Jv/A and Pc is linear over the range of Pc from 15 to 50 cmH2O and its slope is the hydraulic permeability, Lp. Reproducible estimates of Lp can be made in the same venule in separate microperfusions. The value of Pc at Jv/A = 0 varies linearly with perfusate oncotic pressure and is the effective oncotic pressure of the perfusate, sigma delta pi, when the hydrostatic pressure in the pericapillary fluid is zero. The mean value for Lp (+/- S.E.M.) in forty venules was 2.43 (+/- 0.2) x 10(-7) cm s-1 cmH2O-1. Two potential errors of the micro-occlusion technique (vessel distensibility and marker red cell size) were investigated. It was found that the effects of vessel distensibility had little effect on red cell movements at times later than 2 s after a step change in Pc. Red cell size had a potentially large effect on estimates of the absolute values of Lp. Cooling the mesenteric tissues from 37 to 7 degrees C reduced Lp in proportion to the change in the reciprocal of water viscosity with temperature. Rat venular permeability was shown to be sensitive to histamine, with Lp increasing and sigma delta pi falling in a concentration-dependent fashion with histamine concentrations of 1.6 x 10(-5) to 3.3 x 10(-4) mol l-1 in the perfusate.

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