Probably because of methodological problems, little is known about permeability to macromolecules and edema formation in the tracheobronchial microcirculation. A novel atraumatic technique of exposing the guinea pig airway mucosa to mediators has been developed. The intralumenal surface of the trachea and main bronchus is superfused (0.02 ml/min) via a thin plastic tubing introduced into the trachea through the mouth in anesthetized, tilted, and spontaneously breathing guinea pigs. Erythrocytes were labelled in vivo with Technetium 99m and individual hematocrit values and plasma concentrations of Fitc-dextran were determined. By quantification of content of a macromolecular tracer (Fitc-dextran MW 70,000) and blood pool in excised airway tissue the amount of extravasated macromolecules was calculated. In control animals where airways were superfused with saline (0.02 ml/min) during 30 min the extravasated amount of tracer was 0 +/- 14 micrograms/g trachea (n = 15), which was similar to what was found in animals without any superfusion (n = 6). Capsaicin 10(-6) M (total dose 0.1 nmol), probably by activating nerves, produced an extravasation of Fitc-dextran of 223 +/- 32 micrograms/g, n = 8, (p less than 0.005). In guinea pigs intubated for artificial respiration and prepared for vagal stimulation the base-line microvascular values were deranged, and large amounts of macromolecules leaked from the vascular compartment. Still, vagal stimulation in these animals produced further extravasation of macromolecules. It is suggested that atraumatic techniques such as the present superfusion method should be employed in studies of physiological and pharmacological control of permeability to macromolecules in the tracheobronchial microvasculature.