This investigation studies the effects of diets containing varying amounts of linoleic acid (a polyunsaturated fatty acid) and stearic acid (a saturated fatty acid) on tumorigenesis, weight and longevity in strain A/ST mice. Linoleic acid [ 18 carbons and 2 double bonds (18:2)] was chosen to represent a fatty acid known to enhance tumorigenesis and obesity in certain strains of mice. Stearic acid [ 18 carbons and no double bonds (18:0)] represents a saturated fatty acid known to increase the latency period for mammary tumor development and to decrease the rate of tumor growth. This study was conducted to determine whether the effects of fatty acids observed in younger mice on time to tumor, survival and body weights were also found in aging animals. Further, by varying the amount of linoleic acid in the diet, this study examined whether the tumor enhancing effects of increasing amounts of linoleic acid could be overcome by the incorporation of dietary stearic acid. All diets had equal percentages, by weight, of protein, salt, sucrose, mineral salt, and vitamin levels and an equal number of calories per gram of food. The SF diet was rich in linoleic acid. The SA-1 diet contained enough linoleic acid to prevent essential fatty acid deficiency, and the SA-4 diet contained the maximal amount of linoleic acid for tumor enhancement. Total body weight and tumor production in the three dietary groups show a relationship between an increase in body weight and tumor production as the amount of dietary linoleic acid increases. There also is an inverse relationship between animal survival and body weight as the amount of dietary linoleic acid increases. Survival thus appears to be dependent on tumor production in the three dietary groups, where there appears to be an inverse relationship between survival and time to tumor as the amount of dietary linoleic acid increases at each timepoint. These results suggest that the inclusion of stearic acid in the diet can, in part, overcome this enhancing effect of linoleic acid, even at the optimal tumor producing level of linoleic acid. The results of this study indicate that that effects of linoleic and stearic acid in aging mice are similar to those in younger animals. / Department of Biology