The contribution of beta 2-agonist treatment per se and the effect of beta 2-agonists plus allergen exposure was evaluated in two groups of thirteen asthmatic children being treated respectively at sea level during the period of maximal allergen exposure and at high altitude in an environment free of the offending allergens. Bronchial hyperreactivity was evaluated by standardised exercise tests before and after treatment with salbutamol controlled release tablets (4 mg). Challenges were performed at the beginning and after 2 and 4 weeks of treatment. A fourth test was performed 2 days after stopping the treatment. Children treated with salbutamol at sea level (exposure to allergen) showed baseline delta PEF of 16.9 +/- 3.4 and 13.7 +/- 4.2, 20.7 +/- 4.3, 26.0 +/- 5.1 respectively for the second, third and fourth test. Children treated at high altitude showed respectively delta PEF of 34.9 +/- 5.1, 31.1 +/- 4.9, 26.5 +/- 5.4, 27.9 +/- 5.0. These data suggest that oral salbutamol per se is not responsible for an increase in bronchial responsiveness, but eventually suggest that treatment with beta 2-agonists at the same time as continued allergen exposure may be responsible for an increase in bronchial hyperresponsiveness.