While inhaled steroids (IS) are increasingly recognized as having a more rapid onset of action than was once thought, little is known about the early changes in objective measures of respiratory function that follow the inhalation of repeated doses. These early effects were examined in a randomized, double-blind, placebo-controlled, crossover study of 20 children aged 10-16 years with stable mild asthma. Beclomethasone dipropionate (BDP) 2,000 mcg, fluticasone propionate (FP) 400 mcg, and placebo were given twice daily for three doses. Airway hyperreactivity (AHR) to methacholine (PC20), pulmonary function tests (PFT: FVC, FEV1, FEF25-75%), and the rate of recovery from methacholine-induced bronchospasm following administration of salbutamol were determined at 8 h (after 1 dose) and at 32 h (after three doses). At 8 h, minor improvements in AHR were demonstrated, averaging 0.32 doubling doses in PC20. At 32 h, significant improvements in AHR and PFTs were present, averaging 0.92 doubling doses in PC20, 3.96% of predicted values in FEV1, and 7.74% of predicted values in FEF25-75%. No significant changes occurred in FVC. There were no significant differences between the effects of BDP and FP. Inhaled steroids were associated with a slower response to salbutamol following methacholine challenge testing at 32 h. We conclude that IS, given in repeated high doses, result in significant improvements within 32 h in both AHR and PFTs, along with changes in response to beta2 agonists. These effects are likely to be the result of the topical activity of IS.