AbstractExperiments were performed to investigate whether the type of dietary fat might affect metastasis from the 13762 mammary tumor. Female Fischer 344 retired breeder rats were placed into one of five dietary groups: 23% (wt/wt) and 5% (wt/wt) corn oil (HFCO, LFCO), 20% (wt/wt) and 5% (wt/wt) olive oil (HFOO and LFOO), or 20% (wt/wt) beef tallow (HFBT). After four weeks on the diets, each rat had a 2‐mm3piece of the tumor subcutaneously implanted. Primary tumor growth and body weight were monitored weekly for 40 days. At necropsy, the average volume of pulmonary metastases in the HFCO animals (n= 30) was significantly greater than in the other four groups. Among the four groups that did not differ significantly from each other, the rank order in average volume of pulmonary metastasis was as follows: HFOO (n =25), HFBT (n =26), LFOO (n= 25), and LFCO (n= 18). Growth of the primary tumor did not vary appreciably among the five groups despite the significant difference in pulmonary metastasis volume. The diets varied considerably in fatty acid content; the most salient difference was that the HFCO diet, which stimulated metastasis significantly more than the other diets did, contained about four times more linoleic acid (18:2) than the other diets. The relevance of this difference and other fatty acid differences is discussed. These results suggest that the quality of dietary fat can be an important determinant of pulmonary metastasis from the 13762 mammary tumor in retired breeder rats.