Dog experiments were performed to establish the lipid peroxidation of heart tissue (measured by formation of malone-dialdehyde--MDA) and the natural scavenger action (measured by determination of superoxide dismutase--SOD and of reduced glutathione--GSH). Experimental groups were: control dogs having intact heart, dogs ventilated with hypoxic gas (N2O and O2 at a ratio of 10:1) for 1, 2 and 3 hours and dogs having acute coronary ligature for 1, 2, 3 and 24 hours. Acute hypoxia caused a gradual increase of MDA concentration, a moderate increase of the GSH level and a sharp decrease in SOD activity. In ischaemic heart tissue, these changes were very distinctive. High MDA values were found after 3 hours. GSH level and SOD activity decreased continually. Increased MDA formation indicates breakdown of polyunsaturated fatty acids (PUFA) in the membranes, decreased GSH and SOD levels indicate impairment of the natural scavenging, clearly outlining the extent of disintegration of the membrane structure and function due to the effect of toxic free oxygen radicals.