In order to determine the sites of net production and removal of the cyclic nucleotides in plasma, various blood vessels were catheterized in 17 anesthetized dogs and arterial and venous concentrations of adenosine 3',5'-monophosphate (cAMP) and guanosine 3',5'-monophosphate (cGMP) were measured by radioimmunoassay. Aortic cAMP was 30+/-2 nM (mean+/-SE) and cGMP was 13+/-1 nM. There were no significant differences for either cyclic nucleotide between the concentration in the aorta and that in the inferior vena cava, coronary sinus, hepatic vein, and femoral vein. The concentration of cAMP in renal venous plasma was 25% lower than in aortic plasma, and renal venous cGMP was 51% lower than in the aorta. The pulmonary arterial concentrations of cAMP and cGMP were slightly lower than in the aorta. The concentration of cGMP in the superior mesenteric vein plasma was 83% greater than in aortic plasma; the concentration of cAMP in this vessel was only 16% greater than that in the aorta. Superior vena cava concentrations of both cyclic nucleotides were slightly greater than arterial concentrations. THE RESULTS SUGGEST THAT: (a) the kidneys are a major site of removal of both cyclic nucleotides from plasma. (b) The lungs may be a site of net addition of both cyclic nucleotides to plasma. (c) The small intestine is a site of net production of both cyclic nucleotides, particularly cGMP. (d) The liver probably removes cyclic nucleotides from plasma. (e) Since no other organs or regions studied added detectable net amounts of cyclic nucleotides to plasma, and since the turnover of these compounds in plasma is known to be rapid, the production of plasma cyclic nucleotides under basal conditions may well be the result of small net contributions may well be the result of small net contributions from many tissues or bidirectional fluxes between tissues and plasma, or both.