Thirty atopic and 30 non-atopic subjects were identified from a population of 7-8 year old children with current respiratory symptoms. The response of the airways to exercise and provocation by methacholine were compared. In these children, who had symptoms but were not necessarily asthmatic, there was no significant correlation between the two stimuli. The atopic children were, however, significantly more responsive than the non-atopic children to both. For the whole group, odds ratios derived for atopy and for an increased response to methacholine (expressed as a PD20--the dose that caused the forced expiratory volume in one second (FEV1) to fall by 20%--of less than 6.4 mumol/l), a positive exercise test (greater than 15% fall in FEV1), and the presence of asthma were 13.5, 3.3, and 21.0, respectively; that for positive response to methacholine and positive exercise challenge was 1.5. Thus though increased bronchial responsiveness to methacholine and exercise challenge are both associated with a diagnosis of asthma, the association between the two stimuli is complex, and supports the view that they reflect entirely different components of airways dysfunction.