This study was undertaken to reevaluate the effect of ageing on body composition and to compare the effects of exercise and food restriction on body fat content in rats. Body fat content increased progressively between ages 12 and 28 months in sedentary freely eating male Long-Evans rats. A program of 3 h of swimming/day caused body weight to plateau at a level approximately 20% below that of the freely eating sedentary rats, primarily due to a more than 100 g lower body fat content in the swimmers. Lean body mass and protein content decreased progressively with ageing; in the sedentary animals, this decrease was significant by 18 months. The swimming protected against the decline in lean body mass and protein content up to age 18 months, but did not prevent a large decrease in lean body mass between ages 18 and 24 months. Food restriction that kept the sedentary rats at the same weight as the swimmers had its major effect on body fat content, which was approximately 40% lower than in the sedentary freely eating rats at all ages. The differences in body composition between the swimmers and food restricted sedentary animals were relatively small and consisted primarily of a lower body fat content in the swimmers. In conclusion, our results show that lean body mass declines with ageing in rats and that this loss of lean tissue is counterbalanced by an increase in fat. Exercise is highly effective in minimizing the increase in body fat, and may delay the decline in lean body mass, with ageing.