Primary cultures of rat heart cells were used as a model system to study the influence of two antioxidants, vitamin E and ascorbyl palmitate, on biological effects of thermally oxidized fat. The free fatty acid fraction of the distillable non‐urea‐adductable fraction of heated corn oil (HCO) was used as the test lipid; the free fatty acid fraction of fresh corn oil was the control, HCO (100 μg/ml medium) depressed the mitotic index, induced excessive lipid accumulation, and increased the number of pyknotic nuclei in the cells. Addition of extra vitamin E (10 μg/ml medium) reduced the toxicity of HCO by counteracting these changes. In comparison, ascorbyl palmitate (10 μg/ml medium) in the presence of HCO was beneficial in that it produced only a slight increase in the mitotic index. HCO treatment also resulted in reduced levels of linoleic and arachidonic acids in the phospholipid fractions of the cells, and addition of vitamin E or ascorbyl palmitate increased the level of arachidonic acid. The triacylglycerol fraction of HCO‐treated cells showed markedly reduced linoleic acid and increased arachidonic acid. These changes were unaffected by the antioxidant treatments. Vitamin E counteracted the adverse effects of HCO treatment on the rat heart cells. Ascorbyl palmitate only was as efficient as vitamin E in elevating the concentration of arachidonic acid at the membrane level in the presence of HCO.