Plasma leakage in inflammation results from intercellular gaps that form in the endothelium of venules. These gaps and related morphological changes in endothelial cells are not readily seen by light microscopy. In this study we sought to visualize such changes by using the selective binding properties of plant lectins. Acute inflammation was induced in the trachea of pathogen-free F344 rats by injecting substance P intravenously, and 1, 3, or 10 min later the vasculature was perfused with fixative followed by a biotinylated lectin. Lectin binding was localized by avidinbiotin complex-peroxidase histochemistry and viewed in tracheal whole mounts by differential-interference contrast microscopy. The binding patterns of the 20 lectins tested fell into 4 groups. Most of the lectins either bound uniformly to the endothelium of normal and inflamed venules (group 1, e.g., Lycopersicon esculentum lectin) or bound weakly or not at all to venules (group 2, e.g., Maackia amurensis I lectin). The uniform binding of group 1 lectins not only revealed the overall vascular architecture but also made visible intercellular gaps and fingerlike processes at endothelial cell borders in inflamed venules. In postcapillary venules after substance P, the fingerlike processes were present along an average of 32% of the endothelial cell perimeter at 1 min, 25% at 3 min, and 7% at 10 min, compared with a baseline value of 2%. A third group of lectins (group 3, e.g., concanavalin A) bound selectively to focal patches of inflamed venules but bound weakly to normal venules. The fourth group (group 4, e.g., Ricinus communis I lectin) bound preferentially to focal patches in inflamed venules and also bound uniformly to normal venules. The focal binding of group 3 and 4 lectins coincided with sites of plasma leakage marked by extravasation of the particulate tracer monastral blue and was associated with subendothelial components of the vessel wall. We conclude that selected lectins reveal novel features of focal sites of plasma leakage, endothelial gaps, and fingerlike processes at endothelial cell borders in inflamed venules.