Beta 2-adrenergic and anticholinergic drugs have shown an action in modulating bronchial hyperreactivity to various stimuli (chemical, physical, immunological, pharmacological). The aim of our study was to compare the efficacy of a combination of ipratropium bromide and fenoterol (Duovent) with the activity of single drugs in the prevention of histamine-induced bronchospasm. Twenty-six atopic asthmatic subjects were examined in a double-blind trial, during an asymptomatic period, with a FEV1 value not lower than 20% of the predicted normal value. During 4 consecutive days all patients received 2 puffs, respectively, of Duovent (200 micrograms fenoterol + 40 micrograms ipratropium bromide), fenoterol (400 micrograms), ipratropium bromide (80 micrograms) and placebo in a randomized order. PD20 values were evaluated after each drug administration: after 2 h in 16 patients and after 5 h in the other 10 patients. The data were modified to log values, and statistical analysis was performed by two-way analysis of variance. This study showed that Duovent and fenoterol have a protective effect against histamine-induced bronchospasm with a significant increase in the PD20 values 2 h (p less than 0.01) and 5 h (p less than 0.05) after treatment when compared to placebo. Duovent inhalation determined a more protective effect than other drugs but not significantly when compared to fenoterol. Some advantages in the modulation of bronchial reactivity could be seen from using Duovent because with the lower dose of the beta 2-adrenergic drug the same results could be obtained without side-effects.