In a double-blind crossover controlled study, intravenous (IV) or nebulized terbutaline was given to eight patients with moderately severe asthma on two separate days. Four incremental doses of terbutaline were given by each route to establish a maximal effect. Both routes of administration produced significant increases increases in FEV1, FVC, PEFR, MEF50% single-breath TLC, and effective pulmonary blood flow. A decrease in slope of alveolar argon plateau was observed with both routes, but helium responsiveness showed variable changes with no significant or consistent effect seen. There was no significant difference between responses to incremental doses and maximal response apart from pulse rate, which rose during IV treatment. These results showed that the IV route had no advantage in terms of effectiveness or site of action over the inhaled route. Since IV treatment can produce systemic side effects, inhaled bronchodilator therapy should be used as the route of choice.