In an attempt to find the optimal single therapeutic dose of fenoterol inhalant solution administered by compressor-powered nebulization, bronchodilator and side effects of five different doses of fenoterol (0.5, 1.0, 1.5, 2.0, and 2.5 mg) and of placebo were compared with those of the recommended therapeutic dose delivered from a metered dose canister in 16 patients with reversible airway obstruction. The fenoterol (except for the metered dose) and the placebo were given in a double-blind, cross-over manner. In comparison with placebo, all doses of fenoterol produced a significant increase in average values of FEV1, FEF25-75%, FVC, and SGaw and decrease in FRC for five to eight hours. There was a trend for the bronchodilator action to become greater and more prolonged with increasing doses of fenoterol. Compared with 0.4 mg given from a metered dose canister, 0.5 mg of fenoterol delivered by compressor powered nebulization was equally effective in bronchodilator potency. Dose-by-dose comparison with isoproterenol indicates that fenoterol is a more potent and longer lasting bronchodilator and has no significant effect on heart rate and blood pressures. The most common side effects were shakiness or tremor of hands which appeared to be dose-related in terms of incidence and intensity. The results of the present study suggest that 0.5 to 1.0 mg of fenoterol is a suitable single therapeutic dose when administered by compressor-powered nebulization.