We determined the effect of IV administered beta 2-adrenergic receptor agonist clenbuterol on pulmonary function and on the response to histamine in 12 healthy ponies. Measurements were made at base line and after saline solution or clenbuterol was administered IV at a dosage of 0.2, 0.8, or 1.6 micrograms/kg. The dosage of clenbuterol used in each study was unknown to the investigators until all the data had been analyzed. Intravenous administration of saline solution or clenbuterol did not alter base-line pulmonary function significantly. Aerosol histamine administration significantly decreased arterial oxygen tension and dynamic compliance and increased pulmonary resistance, respiratory frequency, and minute ventilation, but had no effect on arterial carbon dioxide tension and tidal volume. The magnitude of change in these variables was unaffected by previous administration of clenbuterol at any of the dosages tested. We conclude that clenbuterol at the dosages tested is not a bronchodilator in healthy ponies and does not exert a significant protective effect against aerosol histamine-induced airway narrowing.