1. In six patients with stable chronic bronchial asthma, the effect of a single oral dose of 0.11 mmol (40 mg) of prednisolone was studied by carrying out repeated measurements of dynamic and static lung volumes, thoracic gas volume and airway resistance and maximum expiratory flow-volume curves. 2. No significant diurnal variation in the measurements performed was demonstrated on the 2 days before prednisolone was given. The administration of placebo tablets produced no significant change. 3. A statistically significant improvement was detectable 3 h after administration of the drug in the group of patients studied. The maximum effect was reached within 9–12 h, after which there was a return towards pretreatment values. 4. The improvement in specific conductance and maximum expiratory flow rate as well as in mid-expiratory flow rate suggests that the relief in airway obstruction occurred in both the large and the smaller airways. 5. The peak expiratory flow rate as measured by a Wright's peak flow meter proved to be as sensitive an index of change as any of the other tests employed.