In the anesthetized intact dog, the intravenous injection of caffeine (10 or 20 mg/kg) caused the following changes: (a) decrease in pulmonary resistance; (b) increase in pulmonary compliance; (c) increase in heart rate; and (d) decrease in blood pressure. In the heart-lung preparation, caffeine induced the first three changes: (a) probably by a direct action on the bronchial smooth muscle; item (b) was related to the decrease in pulmonary vascular resistance; and item (c) was accompanied by an increase in myocardial contractility in the heart-lung preparation. There was no evidence to support the theory that caffeine potentiates the effects of epinephrine on the heart and the lung.