Patients with asthma, exhibiting spontaneous bronchial obstruction, from mild to medium severity, were examined by body plethysmography. Results were expressed as specific airway resistance (SRaw) in preference to specific conductance, the latter leading to errors, in particular in cases with dissimilar baseline values. After the inhalation of a beta-stimulating drug (salbutamol), the decreases in SRaw was directly related to initial SRaw, either in a group (n equal 30) or in individual cases. After the injection of a beta-blocking drug (propranolol), the increase in SRaw, found abnormal in 60% of the subjects, could be related to initial SRaw neither in a group (n equal 40) nor in individual cases. These findings indicate that submaximal bronchial obstruction in patients with asthma is mainly due to a bronchoconstriction, whatever the degree of the obstruction may be. On the opposite, there is an increased bronchodilator adrenergic influence in a majority of patients, not predictable after the degree of initial airway obstruction.