The bronchodilator effect of 0.02 mg of procaterol compared to 0.2 mg of salbutamol as a reference drug administered by single dose inhalation, has been tested in 15 asthmatic patients with a stable clinical situation, using a double-blind study. The clinical value of the administered dose was determined by FEV1, FEV 25-75%, PEF, MEF 50% FVC, SRaw, VC and FRC. These functional tests together with the evaluation of secondary and cardiovascular effects were performed every hour during a 12 hour period. The results revealed that the procaterol aerosol was more potent (although not statistically significant) than the salbutamol, and had a longer activity time. There were no differences in the secondary effects which were mild and transitory. These results suggest that procaterol is a potent bronchodilator, with a long-lasting activity and minimal cardiovascular and adverse side effects.