We investigated the exchange of water and macromolecules across venular microvessels after permeability was increased. Quantitative fluorescence microscopy was used to measure albumin permeability coefficients in individually perfused microvessels of decerebrate frogs. Control permeability coefficient was 2.3 +/- 0.25 X 10(-7) cm/s. Solvent drag increased the apparent solute permeability coefficient (Ps) by 0.57 +/- 0.05 X 10(-7) cm/s for each cmH2O increase in microvessel pressure. The divalent cation ionophore A23187 (0.1-5 microM) produced a transient increase in Ps to a peak value (within 1-3 min), followed (after 4-8 min) by a sustained increase in permeability (16-34% of peak values). Peak values of Ps were 13 and 80 times control for 0.1 and 5 microM A23187, respectively. Both diffusion and solvent drag contributed to the sustained increase in Ps. The equivalent pore radius of the structures determining diffusion and solvent drag was less than or equal to 25 nm during the sustained increase in permeability, smaller than observed gaps between adjacent endothelial cells. The basement membrane and a fibrous matrix secreted by endothelial cells into the gaps may offer resistance to exchange in the high permeability state.