For a sympathomimetic bronchodilator to be effective, it should be transported in an active form, from the place of administration to the receptor site, as well as the kind of medication and its route of administration. It has been shown both in the laboratory as well as in clinical studies that the intensity and duration of the sympathomimetic bronchodilator effect keeps decreasing although the tolerance does not seem to be clinically so important for the new bronchodilators. The beta 2 agonists are used via oral, parenteral and inhalatory route. The rapidity of the action by inhalation makes this method effective to treat intermittent symptoms or during acute exacerbations in bronquial asthma. In this study, 29 patients were included, of which 18 were symptomatic and 11 were asymptomatic asthmatics. Those patients with symptomatic and asymptomatic asthmatics were included in the first and second group respectively. The criteria of exclusion for both groups were: patients with restrictive or non-reversible bronchopulmonary disease, cardiovascular disease, thyrotoxicosis, patients under treatment with steroids and acute infection. The age was lower than 6 years and more than 15 years. A clinical history and a spirometric study (without medication) were performed to determine the basal line values of the forced spirometry volume one second (VEF1) and the vital capacity (CV). An inhalation of fenoterol in aerosol was administered immediately afterwards. At 3, 20, 30 and 60 minutes new spirometries were carried out to determine the initial and the therapeutic effect.(ABSTRACT TRUNCATED AT 250 WORDS)