To investigate whether cessation of regular β-agonist treatment results in an increase in bronchial responsiveness, two double-blind, randomised crossover studies were done. Subjects with mild asthma were investigated to determine the course of change in bronchial responsiveness, measured as the provocative dose (PD₂₀) of histamine that caused a 20% fall in forced expiratory volume in 1 s after short-term and longer term treatment with an inhaled β-agonist. In the first study in 8 subjects, 500 and 2000 μg terbutaline thrice in 1 day protected against histamine-induced bronchoconstriction, and the increase in PD₂₀ compared with placebo remained high throughout the day and overnight. In the second study 8 subjects received placebo or terbutaline 750 μg thrice daily for 14 days. The protection afforded by terbutaline against histamine-induced bronchoconstriction on day 14 was less than that on day 1 by 40% in the morning and 82% in the afternoon. On day 15 PD₂₀ was lower after stopping terbutaline than placebo, with a maximum difference of 1·5 (95% CI 0·6-2·5) doubling-doses of histamine 23 h after the end of treatment. Thus treatment with terbutaline for 1 day did not result in any rebound increase in bronchial responsiveness. Treatment for 2 weeks impaired the ability of terbutaline to protect against histamine-induced bronchoconstriction, and was followed by a rebound increase in bronchial responsiveness after cessation of treatment.