Abstract:The bronchodilator effect of fenoterol hydrobromide (0.5, 1.25, and 2.5 mg) was compared with either isoproternol (2.8 mg) or isoetharine (5 mg) with phenylephrine (1.25 mg) in a double‐blind placebo‐controlled study. When delivered by an intermittent positive‐pressure breathing device to 24 nonsmoking young adult asthmatic subjects, fenoterol produced significant improvement in forced expiratory volume at 1 second (FEV1), in maximum midexpiratory flow (FEF25–75%), and in forced expiratory flow at 25 per cent of vital capacity (FEF25%) for 6 to 8 hours, whereas isoproterenol and isoetharine with phenylephrine produced improvement for 1 and 2 hours, respectively. The lowest dosage of fenoterol was as effective as the highest but had fewer adverse