Microvascular permeability to fluid and hydrophilic solutes is restricted to channels between the endothelial cells and through the fenestrations when these are present. The channels have a molecular filter which appears to be a lattice of fibrous molecules reinforced by plasma proteins. The quantitative description of blood-tissue fluid movements is complicated by plasma protein permeation which leads to a non-linear steady state relation between fluid filtration and microvascular pressure. When fluid filtration is low it can be balanced by lymph flow but at high pressures oedema develops. Microvascular pressure rises less than expected with increments of venous pressure owing to local vasoconstriction of the arterioles. The sluggish flow which results from this vasoconstriction and high venous pressure leads to a haemoconcentration which reduces oedema formation but favours leucocyte and platelet sequestration within the microcirculation.