The bronchodilator response to inhaled fenoterol (400 micrograms) was examined in the morning and in the afternoon before and during oral sustained-release aminophylline treatment in eight patients with chronic reversible airway obstruction. Bronchodilatation was evaluated by measuring serial peak expiratory flow rates (PEFR) for eight hours after inhaled fenoterol and calculating the area under the time-response curves and the percentage increment from the baseline values. The patients showed an enhancement of the bronchodilatation achieved with fenoterol in the morning during aminophylline treatment. In the afternoon, instead, the effect of the fenoterol was not improved by oral aminophylline. This different effect of oral aminophylline might depend on the variable degree of potential reversibility present or diurnal variation in the bronchial response.