The coronary efflux of radioactive3',5'-cyclic adenosine monophosphate (cAMP) and adenosine from isolated guinea pig hearts was measured following selective prelabelling of coronary endothelial adenine nucleotides with 10 nM [2,8,5'-3H] adenosine. Intracoronary infusion of adenosine and its derivatives5'-N-ethyl-carboxamide-adenosine (NECA), (-)-N6-(R-phenyl-isopropyl)-adenosine (R-PIA), and (+)-N6-(S-phenyl-isopropyl)-adenosine (S-PIA) caused dose-dependent parallel increases in both coronary flow and the coronary efflux of radioactive cAMP with a rank order of potency: NECA > R-PIA > adenosine > S-PIA. In contrast, adenosine receptor stimulation of isolated cardio-myocytes in primary culture decreased the cellular release of cAMP below control levels with a rank order of potency: R-PIA > NECA. Under control conditions, coronary efflux of adenosine and cAMP was 34.3 ± 2.3 and 3.9 ± 0.8 pmol/min (mean ± SEM,n= 6), respectively. NECA (12 μM) caused an increase in cardiac cAMP release of 3.8 times and elevated the specific radioactivity of cAMP 5 times to 63.7 ± 6.0 Ci/mol, a value 11 times greater than the specific radioactivity of tissue ATP; Based on these findings, it was concluded that the coronary endothelium possesses adenosine A2 receptors linked to adenylate cyclase, which are activated in parallel with increases in coronary flow and that cardiomyocyte adenosine receptors are predominantly of the A1 subtype. In addition, the contribution of the coronary endothelium to total cardiac adenosine release was calculated to be 14% using the specific radioactivities of adenosine and cAMP released into the effluent perfusate.