1. We have tested the hypothesis that a reduction in endothelial cell volume increases microvessel permeability and that the degree of endothelial cell attachment to their basement membranes determines the magnitude of permeability changes caused by a reduction in endothelial cell volume. 2. A decrease in endothelial cell volume was imposed on both intact microvessels and cultured endothelial monolayers by raising osmolarity by 100 mosmol l-1. 3. We found that hypertonic solutions did not increase the hydraulic permeability (Lp) of individually perfused venular microvessels in frog mesentery when the perfusate contained albumin. Hypertonic solutions did increase Lp, however, after we perfused the microvessels with the peptide Gly-Arg-Gly Asp-Thr-Pro (GRGDTP; 0.3 mmol l-1), to disrupt integrin-dependent endothelial cell (EC) attachment to the extracellular matrix (ECM). 4. After albumin was removed from the perfusate, hypertonic solutions increased Lp of microvessels and the permeability of endothelial monolayers to alpha-lactalbumin. 5. Our findings indicate that endothelial cell integrin-ECM binding plays a role in transducing changes in cell volume and/or shape into changes in permeability. We hypothesize that removal of albumin from the vascular perfusate may compromise EC-ECM interactions via an integrin-dependent mechanism.