Reperfusion of heart muscle after prolonged ischaemia is associated with metabolic and functional abnormalities and eventual cell death. Free radical induced lipid peroxidation of cell membranes is thought to be a major mechanism in the evolution of reperfusion damage. The evidences in support for this kind of damage are based on tissue malondialdehyde quantitation by the thiobarbituric acid test (TBA-test). In an attempt to verify this topic we have subjected isolated and Langendorff perfused rabbit hearts to a period of 60 minutes of severe ischaemia plus 30 minutes of reperfusion. At appropriate time points malondialdehyde was determined in the tissue by means of TBA-test and directly by reversed phase, high pressure, liquid chromatography (HPLC). We have found no correlation between the two compared assays. During reperfusion, there was the formation of non-lipid related, malondialdehyde-like, TBA-reactive substance which leads to overestimations of the extent of lipid peroxidation. On the contrary, by direct HPLC quantitation, there was a decrease of tissue malondialdehyde during ischaemia and during the early phases of reperfusion. Our results demonstrate that TBA-test is not a reliable index of malondialdehyde accumulation in organ system.