Rat and dog experiments were undertaken to describe the range of the endogenous lipid peroxidation (measured by formation of malondialdehyde in heart tissue) and to analyze the effects of hyperoxic, hypoxic and ischemic influences. The acute hyperoxia caused a moderate increase in the lipid peroxidation. The malondialdehyde formation was perceptibly higher in hypoxia, while the highest values were found in ischemic hearts. It can be concluded that the cytotoxic oxygen metabolites cause unfavourable influences on the heart structure and function, for this reason an antioxidant therapy is to be recommended.