A double-blind study was carried out with 100 young men suffering from latent bronchial asthma diagnosed by histamine provocation, and the potency and duration of the protective effect of Alupent® and Th 1165a against histamine-provoked bronchospasm was investigated. Alupent in oral doses of 15 mg and 20 mg, and a dose of 1,500 µgadministered by inhalation (metered aerosol) provided partial protection against histamine provocation lasting 3 h after oral administration and 6 h after inhalation. Th 1165a in all doses was superior to Alupent in its potency and duration of effect. The protective effect of Th 1165a was correspondingly greater than that of Alupent. The total mean values calculated for the whole duration of the experiment for Th 1165a were 73% (vital capacity) and 67.4% (maximum voluntary ventilation), although the dose of each inhalation of Th 1165a was not more than approximately ¼ of the Alupent dose. The effect of the Th 1165a metered aerosol was still statistically significant after 6 h. The potency and duration of effect of the metered aerosols of both preparations were superior to those of the oral doses, but the former exerted less pronounced effects on the heart and circulation and fewer unpleasant subjective side-effects (tachycardia, tremor). This fact indicates that the effect of Alupent and Th 1165a administered by inhalation is primarily loc