Local application of inflammatory mediators to the hamster cheek pouch produces an immediate increase in the number of leaking postcapillary venules as observed by intravital light microscopy. Leaks are illuminated by using fluorescein-labeled dextran given i.v. before mediator challenge. All mediators that have been tested produce a similar pattern of vascular leakage exclusively from postcapillary venules. Mediators can be characterized by their effects on vascular permeability and whether they produce dilation (bradykinin, prostaglandins [PGs]) or constriction (leukotrienes [LTs]) of arterioles. The rank order potency for vascular leakage is LTs greater than bradykinin greater than histamine greater than PGs. A linear regression for the relation between dose of mediator and number of leaky venules has been shown for several mediators, e.g., bradykinin, histamine, and LTs. Inhibition of mediator-induced vascular leakage is produced by a wide variety of substances subsequent to a direct effect on the venular endothelial cell. Morphological, physiological, and pharmacological findings are consistent, and provide evidence for the regulation of macromolecular permeability by the endothelial cells in the postcapillary venules.