The effect throughout the day of inhaled fenoterol on the bronchial responsiveness to inhaled histamine was evaluated in 8 asthmatic patients. The airway response to increasing concentration of histamine aerosol was assessed by measurement of forced expiratory volume in one second (FEV1). The provocative dose of histamine needed to cause a 20% fall in starting FEV1 was calculated from the cumulative log dose response curves. Histamine challenges were performed in duplicate, on separate days, after premedication with placebo or fenoterol given by metered dose inhalers in the morning and in the afternoon. The mean starting FEV1 in the morning and in the afternoon did not change significantly after placebo inhaler, but fenoterol caused a mean increase in FEV1 of 11.11% in the morning and 4.92% in the afternoon. Geometric mean histamine PC20 was significantly higher after fenoterol than after placebo. After fenoterol there was no significant difference between morning and afternoon geometric mean histamine PC20. No relation was found between bronchodilatation achieved by fenoterol and increase in the geometric mean histamine PC20 (p greater than 0.05). There was instead a significant relationship between starting FEV1 and histamine PC20 in the morning and in the afternoon both after placebo and fenoterol.