In asthmatic children it was investigated whether the degree of impairment of the adrenergic response on exogenous stimuli is related to the magnitude of the 24-hour amplitude in airflow obstructions. Urinary-adrenaline and noradrenaline excretion after house dust mite (HDM) inhalation and after exercise was measured. Nine children with (group I), and nine without increased airflow obstruction overnight (group II) and nine age matched healthy children (group C) were included in the study. All patients showed an early obstructive reaction (EOR) after HDM challenge. Six children in group I and five in group II developed an EOR on exercise. A Significant increase in urinary adrenaline excretion was observed after exercise in the control group (P<0.05, values on the control and challenge day being 5.4±0.9 and 10.0±1.6 μmol mol creat.). The same occurred for noradrenaline (P<0.01, values being 28.2±2.5 and 49.0±5.7 μmol mol creat.). Adrenergic response after both stimuli was impaired in the asthmatic groups, in group I more pronounced than in group II. Values from group I for adrenaline on the control day. HDM and exercise challenge were 6.0±0.8, 4.7±0.6, 6.0±1.0 and for noradrenaline 36.1±2.7, 27.2±2.3, 38.4±4.9 μmol mol creat., respectively. Values from group II for adrenaline on these days were 5.6±1.0, 3.7±0.6 and 9.0±1.3 and for noradrenaline 28.3±3.2, 22.4±2.5, 41.3±5.9 μmol/mol creat., respectively. The results of this study suggest that the degree of impairment of the adrenergic response on exercise in asthmatic children is related to the magnitude of the 24 hr amplitude in pulmonary function.