The development of drug tolerance in two groups of asthmatics treated with beta-adrenergic bronchodilators was studied by respiratory function methods. During one-year treatment with conventional therapeutic doses of selective beta-2-receptor stimulant aerosols, none of the patients showed subsensitivity to the bronchodilatory effect of terbutaline. Dose-response curves were plotted upon the inhalation of salbutamol in two-week intervals in patients treated with betamimetics. There was no decrease in the airways beta-adrenergic receptor function as compared to the untreated control group. The results show that prolonged treatment with therapeutic doses of inhaled beta-adrenergic bronchodilators does not result in drug tachyphylaxis, which is in accord with the clinical experience that there is no loss of effect of the preparations during their continuous administration.