A technique for measuring pulmonary resistance to airflow without the interference of compliance changes was used to obtain data on the bronchodilator activity of ephedrine sulfate and methoxyphenamine hydrochloride. The resting pulmonary resistance of cats was increased by injecting histamine (100 mcg., base, i.v.) or 5-hydroxytryptophan creatinine sulfate (25 mcg., base, i.v.) or by electrical stimulation (5 v., 1.5 msec., 15 c.p.s.) of the peripheral ends of both vagus nerves. The average maximum per cent inhibition of the increase in pulmonary resistance induced by the constrictor agents was used to calculate ED50's of the test drugs. Drugs were administered either intraduodenally or intravenously. The ability to obtain valid quantitative data for the evaluation of potential bronchodilator agents offers a distinct advantage over the methods presently employed.