1. To determine whether an autonomic nervous system imbalance might underlie the nocturnal dyspnoea in patients with chronic airflow obstruction (CAO), we determined FEV1, sinus arrhythmia gap (SA gap), heart rate and urinary adrenaline and noradrenaline excretion every 4 h over 24 h. Measurements were performed in eight non-allergic patients with CAO and eight age- and sex-matched normal controls. 2. The amplitude of the circadian changes in FEV1 in patients and controls was 27 ± 2% and 7 ± 1% respectively (P < 0.001). 3. Both an increased SA gap and a decreased heart rate are features of increased vagal activity. This vagal activity was significantly increased in patients, compared with normal controls (difference P < 0.01), the difference being maximal at night. This increased activity might contribute to a bronchial obstruction in these patients. 4. Urinary adrenaline excretion was significantly higher by day than by night in both patients and normal controls (P < 0.01). The urinary levels of adrenaline in the patients were significantly decreased at all hours of observation as compared with levels in normal controls (P < 0.05). 5. Urinary noradrenaline levels were significantly lower in patients as compared with normal subjects (P < 0.01), and lower by night than by day. 6. Urinary histamine and Nτ-methylhistamine excretion were in the normal range in each individual. Urinary levels, however, were significantly higher in patients at all hours of observation (P < 0.05). No circadian rhythm was shown. Plasma cortisol levels showed a normal circadian variation, similar in patients and normal subjects. 7. Bronchial constriction in patients with CAO may be explained by an autonomic nervous system imbalance. Arguments favouring this hypothesis were the findings that CAO patients, in contrast with normal controls, showed lower urinary adrenaline and noradrenaline excretion throughout with a fall at night, and a higher vagal tone with an increase at night. Moreover, a higher histamine and Nτ-methylhistamine excretion both day and night existed in the patient group. A combination of these factors may lead to the nocturnal dyspnoea in patients with CAO.