Adenosine and inosine are believed to have cardioprotective effects. However, little is known about their possible role in the metabolic autoregulation of human coronaries and in pathologic conditions with supply/demand imbalance of the heart such as coronary artery disease. Since these low molecular weight nucleosides freely diffuse through the monolayer of the visceral pericardium, adenosine and inosine concentrations in pericardial fluid may well reflect the conditions in cardiac interstitium. The pericardial fluid and systemic venous blood adenosine and inosine concentrations were measured in 98 human subjects undergoing heart surgery for coronary artery disease or valvular heart disease. Adenosine and inosine concentrations were measured by HPLC with UV detection. In subjects with coronary artery disease pericardial fluid nucleoside concentrations were significantly higher than in patients with valvular heart disease (adenosine: 1545 (996-3146) nmol/L [median (25th-75th quartiles)] vs. 738 (390-2527) nmol/L, P<0.01; inosine: 658 (321-1331) nmol/L vs. 347 (159-1037) nmol/L, P<0.05), while in both patient groups pericardial fluid nucleoside concentrations were higher by an order of magnitude than in venous plasma. Our results show the enhanced release of adenosine and inosine by the ischemic myocardium as a marker of supply/demand imbalance and support the hypothesis that these cardiac nucleosides may have an important role in the adaptation of coronary blood flow in human coronary artery disease.