Autooxidation of polyunsaturated fatty acids (PUFAs) of edible oils results in the formation of fatty acid hydroperoxides that can undergo further chemical transformations to yield a variety of re-arranged and chain-cleavage products. Since the oxidation products of PUFAs have been reported to have cytotoxic and mutagenic effects, the consumption of rancid oils and fats represents a possible health hazard for the population. Storage of corn oil at room temperature and in the refrigerator for a forty-eight month period resulted in two different qualities of oil samples, which were characterized by UV, titrimetric (peroxide value, acid value) and GC-MS methods. Earlier it was demonstrated that the increase of expression of certain oncogenes and tumor suppressor genes is a method of choice for the early detection of carcinogen exposure. Treatment of CBA/Calpha inbred mice with the two oil samples showed significantly increased expression of the Ha-ras gene in all the investigated organs (liver, lung, kidney, thymus and spleen) of the rancid corn oil-treated animals. Expression of the c-myc and the p53 genes was also increased after the rancid corn oil-treatment in all the organs but the thymus of the mice. The results suggest that rancid oils, rich in omega-6 unsaturated fatty acids, could be involved not only in tumor promotion but in initiation as well.