We examined the effect of salmeterol, a long-acting beta2-adrenergic receptor agonist, on ovalbumin-induced plasma leakage and leukocyte adhesion in tracheal blood vessels of unanesthetized brown Norway rats. Ovalbumin challenge of sensitized rats resulted in both early and late phases of plasma leakage as measured with Evans blue. The early phase occurred during the challenge. The late phase began 2 h after the challenge, peaked at 4 h, and ended by 24 h. Ovalbumin challenge also increased the number of adherent leukocytes in mucosal blood vessels at 4 h. Nebulized salmeterol (5 mg/ml for 10 min) administered before the challenge inhibited the early-phase leak by 26%, 58%, or 85%, depending upon whether the interval between pretreatment and challenge was 0.5, 1, or 4.5 h, respectively. Similarly, salmeterol pretreatment (0.05 to 5 mg/ml for 10 min) reduced the late-phase leak at 4 h. Late-phase leakage was completely blocked at concentrations of 0.5 mg/ml or greater. This inhibitory effect was prevented by prior injection of the beta2-adrenergic receptor antagonist ICI-118,551. Salmeterol pretreatment also reduced the adherence of neutrophils, eosinophils, monocytes, and lymphocytes in mucosal blood vessels at 4 h. We conclude that salmeterol pretreatment can reduce the plasma leakage and leukocyte adhesion in early- and late-phase responses after antigen challenge through its action on beta2 receptors.