Isocapnic hyperventilation dose response curves were constructed for 11 asthmatic children before and after pretreatment with placebo or ipratropium bromide, 40-1500 micrograms given by inhalation, on three separate days. The response before and after placebo was highly reproducible (within subject coefficient of variation 7.5%, 18%, and 22% for intervals of two hours, within two weeks, and over two weeks). It was independent of baseline lung function. Complete protection against hyperventilation induced asthma was achieved by ipratropium bromide 40 micrograms in six children and by 200 micrograms or more in a further four. The remaining child was unaffected by any dose of ipratropium up to 1500 micrograms. The dose of ipratropium required for protection was better related to the subjects' requirement for regular medication than to their sensitivity to hyperventilation or baseline lung function.