A radiotracer technique is described which enables direct measurement of the dose and distribution of inhaled aerosol bronchodilator in man. The mean (+/-SD) amounts of the B2-adrenergic agonist, fenoterol, administered to a group of 12 asthmatic subjects in a double-blind randomized fashion were: placebo, 0 microgram; low dose, 5.6 (+/-1.2) microgram; medium dose, 32.7 (+/-7.3) microgram; and high dose, 127.5 (+/-29.2) microgram, with a mean of 86.3% of the total subject dose being deposited in the lungs. The medium and high doses of fenoterol produced similar increases above baseline in forced expired volume in 1 sec (FEV1), maximum flow at 50% of vital capacity (V max 50), and maximum flow at 25% of vital capacity (V max 25). These increases were greater than those with placebo for the entire 4-hr study (p less than 0.01). The low dose of fenoterol was more effective than placebo in increasing FEV1, V max 50, and V max 25 above baseline values (p less than 0.05), but not for the entire 4-hr study. The high-dose fenoterol caused palpitations and tremor in 3 of the 12 subjects, and the medium-dose fenoterol caused palpitations in one of these subjects.