The tolerability and the duration of effect of 12 micrograms of formoterol and 25 micrograms of procaterol administered via metered-dose aerosol to 12 stable asthmatic patients were evaluated in a double-blind, placebo controlled trial. FEV1, pulse rate, and blood pressure were measured at baseline and every two hours after dosing for 12 hours. The bronchodilation peak was observed after two hours for both drugs. Formoterol induced a significant bronchodilating effect for 12 hours compared with both baseline and placebo values. With procaterol, significant bronchodilation occurred for six hours compared with baseline values and four hours compared with placebo. No significant changes were observed in pulse rate and blood pressure with either drug. Four subjects complained of muscle tremor after procaterol administration. We conclude that in subjects with stable asthma, inhaled formoterol at a dose of 12 micrograms maintains significant bronchodilation for 12 hours after dosing and is very well tolerated. Further studies are required to evaluate effectiveness and tolerability of high dose formoterol treatment in acute severe asthma therapy.