1. Plasma levels of atrial natriuretic peptide (ANP) were measured in seven patients with obstructive sleep apnoea (OSA) while they were awake, during repetitive apnoea and during treatment with nasal continuous positive airway pressure (CPAP). 2. ANP levels in both pulmonary artery and peripheral venous samples were elevated during apnoeic sleep and reduced when apnoea was prevented by nasal CPAP. Mean values of pulmonary artery ANP were 116.3 +/- 17.9 pg/ml during apnoea and 64.8 +/- 15.2 pg/ml (P less than 0.05) on nasal CPAP. 3. It is concluded that there is increased ANP release during sleep in patients with OSA and that CPAP treatment normalizes ANP secretion. These findings may explain previously identified urinary abnormalities in OSA.