To evaluate atrial natriuretic factor (ANF) secretion during sleep in obstructive sleep apnea syndrome (OSAS), plasma ANF was measured every 3 hours before and after effective nasal continuous positive airway pressure (CPAP) treatment in 10 patients with moderate to severe OSAS and 10 normal subjects. The results showed daily changes in ANF levels in normal controls and in OSAS patients after effective therapy, with a nadir at 0300 hours and a peak at 2100 hours. There was no significant daily variation of ANF levels in patients with OSAS before therapy, and ANF levels from midnight to 0900 hours were significantly higher before, as compared with after, therapy. These results indicate that OSAS patients have abnormal ANF secretion. Effective nasal CPAP therapy led to normalization of ANF secretion during sleep.