Urinary excretion and plasma concentrations of adenosine 3′,5′-monophosphate were determined in idiopathic edema patients both at rest and after assumption of the upright position. Patients and normal subjects responded similarly to upright posture with decreases in urinary volume and creatinine excretion and their urinary excretion of cyclic AMP was not significantly different. The plasma concentration of cyclic AMP was high in patients, both in the recumbent and upright positions, and its renal clearance (unlike that of creatinine) was low. Plasma cyclic AMP increased in response to upright posture both in patients and normal subjects. Even if the significance of elevated plasma cyclic AMP is unknown, its presence without an increase in the urinary cyclic AMP suggests that the renal handling of cyclic AMP is abnormal in idiopathic edema; this may be related to an excessive β-adrenergic tone.