When inflammatory stimuli are applied on the airway mucosa, plasma is promptly extravasated from the subepithelial microvessels. The plasma exudate distributes in the lamina propria and much of it is soon transmitted across the epithelial lining. The rapid luminal entry of large plasma solutes must reflect a dramatic change in mucosal permeability. Previously it has been thought that such a perviousness of the mucosal barrier would be bidirectional in nature. This study in anaesthetized guinea-pigs examines whether absorption across the mucosa is increased (above control) during, and immediately after, the plasma exudation process. An oral catheter, introduced into the tracheal lumen, was used to superfuse the lower airways with 0.04 ml of a solution containing the absorption tracer 131I-albumin and a selected dose of a provocating agent: allergen, 3 pmol (ovalbumin in IgE-sensitized animals); bradykinin. 5 nmol; capsaicin, 0.4 nmol; or carbachol, 8 nmol. The superfusate had a desired distribution on the tracheobronchial mucosa so that specific airway and not bronchoalveolar exudation: absorption ratios could be determined. In separate experiments it was confirmed that the present provocations, except carbachol (P > 0.05), moved significant amounts of plasma into the airway lumen (P < 0.001). This was distinctly an increase in the outward mucosal permeability because the absorption of luminal 131I-albumin into circulating plasma was not significantly different from control with any of the provocations (P > 0.05). The present data support our notion that unfiltered plasma exudates can operate on the mucosal surface, in first-line defence reactions, without compromising the integrity of the epithelial lining as a barrier to luminal material.