SUMMARYThe steady-state flow of fluid across the wall of the isolated rabbit common carotid artery has been measured in the presence and absence of flow within the lumen of the vessel. The perfusate solution contained either 10 or 40 mg ml-' albumin and transmural flux was measured by monitoring the rate of movement of fluid into a chamber enclosing the artery. Vasomotion was minimized by the inclusion of the vasodilator sodium nitrite in both the perfusate and the outer bathing solution. A relatively slow luminal flow caused a reversible increase in the transmural flux by 20 30 % relative to the value in the absence of flow. The mechanism responsible for the increasc is not clear, but since it was not affccted by the H1 antagonist, mepyramine, it would not appear to have been mediated by histamine release.