Terbutaline, a new bronchodilator acting on beta-adrenergic receptors, was given to 10 asthmatic patients, who received on separate days 5 mg orally, 10 mg orally, and 0.25 mg subcutaneously. The ventilatory response was assessed by measurement of the FEV(1) before and at intervals after administration. The cardiovascular response was assessed by measurement of the heart rate and blood pressure and by electrocardiography at the same times as spirometry was performed.The ventilatory response to all three doses and by both routes was satisfactory. The maximal increase in FEV(1) after 5 mg orally was only slightly less than that after 10 mg. The maximal increase in heart rate after 5 mg orally was about half that which occurred after 10 mg. It is concluded that 5 mg orally and 0.25 mg subcutaneously are suitable doses.In general a modest fall in blood pressure affected the diastolic more than the systolic. On E.C.G. the T wave was often depressed, and in one patient, it was inverted. A trough-like depression of the QRS baseline occurred several times. The significance of the E.C.G. changes is uncertain.