This study reports the first results of measurements of filtration coefficient (Lp) and osmotic reflection coefficient to sucrose (sigma suc) in single brain microvessels. 2. Microvessels on the surface of frog brain were cannulated with a micropipette and perfused with an artificial cerebrospinal fluid (CSF) containing the low molecular weight impermeant dye carboxyfluorescein (MW 376). The superfusing solution was a similar CSF which could be made hypertonic by the addition of 40-125 mmol l-1 sucrose. 3. Vessels were assessed for dye retention using video-intensified microscopy after occlusion with a glass microneedle. Only six vessels out of a total of ninety-five were tight under the experimental conditions used. Those vessels which were tight were occluded while an osmotic load was applied across them. When this load was 50 mosmol l-1 and less, the steady-state dye concentration within the vessel lumen was similar to that predicted assuming the endothelium behaves as a perfect semipermeable membrane, with concentration polarization of solute. 4. The product Lp sigma was estimated in two ways: (i) from the fitted monoexponential function that described the rising dye concentration within the occluded segment, and (ii) from the initial rate of increase in dye concentration. The two values obtained were similar and it was concluded that sigma NaCl = sigma suc = 1, and the best estimate for filtration coefficient Lp = 2.0 x 10(-9) cm (cmH2O s)-1. 5. At the osmotic loads of 100 mosmol l-1 and more, the initial rate of increase estimate of Lp sigma was less than half of the whole curve estimate, the axial dye distributions were dissimilar from those predicted by a mathematical model based on the perfect semipermeable membrane, and the steady-state concentration was less than 70% of that expected. These findings are consistent with a diffusive pathway having opened. The model was modified to include patches of vessel wall which had developed leaks and a good fit to the data was obtained with a sucrose permeability and an Lp similar to skeletal muscle endothelium. 6. The possibility that water passes through a paracellular pathway across the intact blood-brain barrier is discussed. It is concluded that this pathway could not be detected by the methods used and can carry no more than 50% of the water driven by a hydrostatic pressure gradient.
