T his study was designed to elucidate the mechanism of elevation of plasma cyclic AMP in uremic man. Plasma cyclic AMP was measured in 15 normal subjects and in 18 patients with severe renal failure. In some members from both groups the kinetic parameters of the metabolism of extracellular cyclic AMP were measured. Plasma cyclic AMP was elevated from 23 nM in control subjects to 59 nM in uremic patients, regardless of the presence or absence of the kidneys or parathyroid glands. A single pass of uremic blood through a Kiil hemodialyzer decreased plasma cyclic AMP from 58 to 30 nM. The clearance of cyclic AMP by the dialyzer correlated directly with the blood flow passing through the machine. Hemodialysis for 6 h decreased plasma cyclic AMP levels in the systemic circulation by only 12%. Studies with tritiated cyclic AMP revealed a plasma clearance rate of 624 ml/min in normal subjects and of 344 ml/min in patients with uremia. Such a large decrease in plasma clearance rate cannot be explained by a failure of urinary excretion of cyclic AMP and suggests impairment of "metabolic clearance." In addition, the "plasms production rate" of cyclic AMP was 65% higher in patients with renal failure than in normal subjects. It is concluded that the elevation of plasma cyclic AMP in uremic man is due to a combination of: (a) lack of urinary excretion, (b) decreases metabolic clearance, and (c) increased production of plasma cyclic AMP.