1. Patients with obstructive sleep apnoea have increased diuresis during sleep, which decreases with nasal continuous positive airway pressure treatment. These changes have been attributed to an increased release of atrial natriuretic peptide in obstructive sleep apnoea, and its decrease with continuous positive airway pressure treatment. 2. In order to clarify the change in plasma atrial natriuretic peptide level and to investigate the underlying mechanisms, blood samples were taken at 10 min intervals from nine patients with obstructive sleep apnoea during two nights when the patients were either untreated or treated with continuous positive airway pressure. Polysomnographic monitoring, including transcutaneous oximetry, and measurement of oesophageal pressure were performed simultaneously. Plasma arginine vasopressin was also measured. 3. The plasma level of arginine vasopressin did not change. The level of atrial natriuretic peptide was high and exhibited secretion bursts in six out of the nine patients; it drastically decreased with continuous positive airway pressure treatment. 4. Across the patients, the mean plasma levels of atrial natriuretic peptide was correlated with the degree of hypoxaemia and the degree of oesophageal pressure swings during the sleep apnoeas. 5. Within the patients, cross-correlation studies suggested that the atrial natriuretic peptide secretory bursts were related either to the oesophageal pressure swings or to the apnoea-related hypoxaemia. 6. We conclude that release of atrial natriuretic peptide decreases with continuous positive airway pressure treatment in those patients with obstructive sleep apnoea who have increased release of atrial natriuretic peptide before treatment.(ABSTRACT TRUNCATED AT 250 WORDS)