Background: Pericardial fluid obtained at pericardiocentesis is often subjected to biochemical analysis, and interpreted according to Light���s criteria for pleural effusions. There is little data regarding the normal composition of the physiological pericardial fluid. Methids: Pericardial fluid from 10 patients undergoing elective heart surgery (3 coronary bypass , 3 ASD closure, 2 valvular replace, 2 Aortic dissection repair) was collected. Patients were excluded if they had known pericardial disease, or acute inflammation status, or affecting systemic disease. The biochemical and hematological parameters of the fluid were determined using standard laboratory techniques, and compared with the results obtained for concurrently drawn blood. Results: The median age of the study population was 53.9짹11.3 years. The biochemical analysis of pericardial fluid showed that the concentration of small molecules (glucose, chloride) was similar with serum concentration. In contrast, pericardial fluid lactate dehydrogenase (LDH) level was averaging 1.70 times the serum level, and the mean protein level was 0.53 times of the serum level. Coclusions: The composition of the physiologic pericardial fluid is remarkable for high LDH and protein content. Light���s criteria for pleural effusions (fluid:serum protein ratio that is greater than 0.5, a fluid:serum LDH ratio that is greater than 0.6, or a fluid LDH level that is more than 2/3 of the upper limit for serum levels) may not applicable for differentiate exudative from transudative in pericardial effusion.