High fat diets increase body fat stores. The following experiment was undertaken to determine whether the type of dietary fat could influence fat storage and whether voluntary exercise could prevent diet-induced obesity in mice fed high fat diets. Sixty-nine 6-wk-old female mice were fed one of three diets: low fat (11.5% of energy from fat), beef fat (40.8% of energy from fat) or canola oil (40.8% of energy from fat). In each diet group, 13 mice had free access to activity wheels in their cages (exercising), and the remaining 10 mice were housed in standard mouse cages (nonexercising). Body weight and body composition were measured before and after 8 wk of treatment. The nonexercising mice fed beef fat weighed more and had significantly more body fat (23.2 +/- 2.5 g/100 g body wt) than mice fed the low fat or canola oil diet (13.9 +/- 1.7 and 16.8 +/- 1.9 g/100 g body wt, respectively). Voluntary exercise did not affect lean body mass but did result in significantly lower body fat in all diet groups (beef, 12.6 +/- 0.9; low fat, 7.4 +/- 0.6; canola oil, 9.6 +/- 1.4 g/100 g body wt). The amount of body fat of mice fed the monounsaturated canola oil was significantly less than that of mice fed the beef fat diet, suggesting that the type of fat as well as the amount of fat influences body fat stores. Furthermore, voluntary exercise decreased body fat in all mice and prevented diet-induced obesity in mice fed diets high in fat.