The present study investigated whether dietary fat increases aggressive behavior in male mice and rats. High fat consumption may elevate circulating estrogen levels and estrogens, in turn, are associated with various non-reproductive behaviors, such as male aggression. The animals were assigned to two groups including those consuming a diet high in polyunsaturated fats (43% calories from fat) and those consuming a low-fat diet (16% calories from fat). Each male animal was housed with two females for three weeks. The male mice and rats were then confronted with an intruder kept on a medium-fat feed. The latency to first aggressive encounter was significantly shorter among the male animals kept on a high-fat diet than those males kept on a low-fat diet. Furthermore, the time spent exhibiting aggression was longer in the high-fat groups. Serum levels of estradiol (E2) were elevated by 2-fold in the male animals consuming a high-fat diet, when compared with the male animals kept on a low-fat diet. These findings suggest that dietary fat can increase aggressive behavior in male mice and rats, possibly by elevating circulating E2 levels.