The effects of celiprolol and bisoprolol on cardiopulmonary function in serotonin-infused cats were compared. Celiprolol reversed the bronchoconstrictive effect of serotonin at doses greater than or equal to 1.0 mg/kg. Also, decreases in mean arterial pressure and heart rate were noted after administration of 3-10 and 10 mg/kg celiprolol, respectively. In contrast, bisoprolol tended to induce bronchoconstriction. Reductions in mean arterial pressure and heart rate were observed with 1 or 3 mg/kg. Bisoprolol administration at 10 mg/kg was lethal. The unique ability of celiprolol to induce bronchodilation enhances its therapeutic potential.