An increase in vascular permeability was observed in the middle ear mucosa of squirrel monkeys and guinea pigs after experimentally inducing serous otitis media by mechanically blocking the eustachian tube. Vascular leakage was assessed by the carbon particle-vascular labeling technique. Obvious and intense vascular labeling was seen first 24 hours after surgery, reaching its maximum between one and two days and continuing up until a month with somewhat less visible evidence of intensity. This demonstrates the important role the small vessels play in acute and chronic serous middle ear effusions. Vascular labeling was also demonstrated within minutes after the induction of serum transudation by applying suction directly to the middle ear. In addition, considerable numbers of mast cells were found in the subepithelial tissues of normal animals and their possible functional relationship was discussed.