Fats contained in food products undergo various transformations under the natural conditions. Hydrolysis and auto-oxidation are ranked among them. The hydrolysis of fats causes incomplete acylglycerides and free fatty acids to form. The objective of this study was to determine the impact of water activity on the hydrolytic and oxidative stability of fat derived from corn grains. The samples of corn grains were stored in the atmosphere characterized by three levels of water activity: 0.328; 0.753; and 0.903, at a temperature of 25ºC, during a three month period. After each of the storage months, the fat was extracted and the parameters: acid value, content of polar fractions, and oxidative stability were determined. No matter what water activity was, storing the corn grain caused that, in the extracted fat, the contents of free fatty acids and polar fractions increased, and the oxidative stability decreased. The highest hydrolytic and oxidative changes occurred in the fat isolated from the corn grains stored in the atmosphere showing the highest water activity among all the atmospheres studied.