1. We have investigated the effects of a standardised mixture of hydroxyethyl rutosides (HR) upon the permeability of the walls of single capillaries and venules of the frog mesentery. 2. In each experiment a single vessel was perfused via a micropipette with frog Ringer solutions containing bovine serum albumin (10 mg ml-1) and Ficoll 70 (40 mg ml-1) first in the absence of HR and then with HR added to the perfusate. The permeability of the vessel walls was assessed during each perfusion by using a development of the Landis micro-occlusion technique to estimate their hydraulic permeability (Lp) and the effective osmotic pressure (sigma delta pi) exerted across them by the perfusate macromolecules. 3. Measurements were made both in vessels which appeared to be healthy and in vessels showing signs of stasis or inflammation before perfusion. 4. HR at concentrations of 1.0, 0.1 and 0.01 mg ml-1 reduced hydraulic permeability to approximately half of its value in the absence of HR. It increased sigma delta pi to macromolecules at concentrations of 10, 1.0, 0.1, 0.01 and 0.001 mg ml-1. The effects of HR upon permeability were not reversed within 10 min of perfusion with an HR-free solution. 5. Ultrastructural examination of a number of vessels in which initial high values of permeability were reduced to values within the normal range of permeabilities by HR, showed clear signs of damage to the endothelium, with large gaps between adjacent endothelial cells. 6. These observations suggest that HR does reduce microvascular permeability both in healthy vessels and vessels showing of inflammation. The reduction in permeability of inflamed vessels does not appear to be the result of closure of the gaps between adjacent endothelial cells.