Obstructive sleep apnea (OSA) patients have increased diuresis and natriuresis during sleep. In order to investigate the possible mechanisms of these changes in renal function, 35 consecutively diagnosed OSA patients were studied during sleep before and during nasal continuous positive airway pressure (CPAP) treatment, and were compared with 23 non-snoring controls. The excretion of urine and of electrolytes was increased before treatment and normalized with nasal CPAP treatment. The mechanism involved seems to be decreased sodium reabsorption at the level of the ascending limb of the loop of Henle. The observed increase in cyclic guanosine monophosphate excretion supports the hypothesis of increased atrial natriuretic peptide release during sleep in OSA patients.