Skeletal muscle tremor is the most common side effect when giving the relatively selective .beta.2-adrenergic-stimulators. Tachycardia is a side effect which limits dosage in a few patients only. Dose-response relation was tested on ventilatory capacity, heart rate, blood pressure and skeletal muscle tremor after terbutaline was given i.v. and as an aerosol. Terbutaline infusions in doses exceeding the recommended therapeutic level did not produce maximal relaxation of the bronchial muscle in patients with endogenous asthma, but increased the heart rate by 25 beats/min and more than doubled the tremor. The tachycardia is mainly due to peripheral vasodilation and reflexogenic heart stimulation. Terbutaline given by inhalation produced the same bronchial relaxation without any effect on heart rate, blood pressure or tremor, indicating a local effect. The acute margin of safety was notable; the increase of pulse after 63 inhalations of this long-acting substance being only 16 beats/min.