From the clinician's standpoint, an ideal bronchodilator should have (1) selectivity for bronchial smooth muscle, (2) minimal stimulatory effects on the cardiovascular and central nervous system, (3) prolonged action, (4) rapid onset of action, and (5) oral effectiveness for patient convenience. In addition, bronchodilation should occur in both central and peripheral airways without causing worsening of gas exchange due to aggravated ventilation perfusion inequalities. The observations reported by Banner et al in this issue (see p 434) again raise questions regarding the cardiovascular side effects of the different bronchodilator agents that are currently in clinical use. While differences may exist among various bronchodilators with respect to their effects on respiratory mechanics and gas exchange, the search for new compounds has primarily resulted from the need for agents with prolonged duration of action, convenient mode of administration, and fewer cardiovascular side effects. The increase in alveoloarterial oxygen tension difference after administration