To determine whether the nocturnal decrease in arginine vasopressin (AVP) secretion into the plasma, found in a patient with multiple system atrophy (MSA) reported previously, is a usual finding in MSA, the plasma AVP concentrations in 13 patients with MSA were measured every 4 hours during a 24 hour period. The plasma AVP concentrations in these patients showed significant daily variations and were the lowest during the night. This finding indicates that patients with MSA often exhibit nocturnal decrease in AVP secretion into the plasma. The results suggest the possibility that the system responsible for the daily variations in AVP secretion is involved in MSA.