Effects of fat intake on body composition and lipogenic enzyme activities were examined in hamsters. Forty-two female hamsters were randomly divided into two groups of 21 each. One group had access to voluntary disc running; the other group was sedentary. All were fed a low-fat, high-carbohydrate diet. After 30 d, seven hamsters from each group were killed and all running activity was discontinued. Remaining hamsters were separately assigned to two groups of seven and fed either a low-fat or high-fat diet for 7 d and then killed. Voluntary running increased food intake and weight gain and reduced body fat (52%). Cessation of exercise greatly enhanced body fat gain. High-fat diet enhanced weight gain. Increased energy intake during postexercise period appeared to cause enhanced body fat gain. The high-fat diet suppressed lipogenic enzyme activities. Serum triglyceride, total cholesterol, and HDL cholesterol levels were not affected by exercise or cessation of exercise.