Summary Elevated nocturnal plasma atrial natriuretic peptide (ANP) levels were found in patients with obstructive sleep apnoea (OSA). The purpose of our study was to examine the secretion of ANP during the night and to measure changes in oxygen saturation, pulmonary artery pressure and intrathoracic pressure swings in patients with OSA. Moreover, we analysed the secretion of ANP and the pulmonary artery pressure in different behavioural states, e.g. awake, at exercise and asleep. Consecutive apnoeas in non-rapid eye movement (NREM) sleep at the beginning, middle and end of the sleep study were analysed in six patients with obstructive sleep apnoea. In addition, we measured the plasma levels of ANP. The apnoea duration was significantly longer (P< 0.05) at the middle of the sleep study than at the beginning or end. Correspondingly, the end-apnoeic oxygen saturation and end-apnoeic oesophageal pressure were both significantly lower (P< 0.05) in the middle of the sleep study than at the beginning or end. No significant differences were found in the end-apnoeic systolic transmural pulmonary artery pressure (PPA™) and the levels of ANP. Evaluation of the ANP levels during different behavioural states revealed that the asleep levels were slightly, but not significantly, higher than the awake levels (0.235±0.088 vs. 0.207±0.057 nmol/L). However, the highest levels were found during exercise (0.334±0.170 nmol/L) with a significant difference compared with the awake and asleep levels. These data suggest that volume effects may be a potent factor in liberating ANP during exercise, but the role of OSA in ANP secretion when asleep is questionable.