This study was designed to investigate age-related changes in body composition and cell mediated immunity. Male Sprague-Dawley rats were fed stock diet ad libitum and 12 rats were sacrified at 1, 4, 6, 12 months of age. Body weight Increased sharply from 1 to 4 months. and Increased steadily thereafter. The weights of liver, epididymal fat pads, and kidney increased in similar pattern as body weight, but their relative ratio to body weight decreased with age. The ratio of epididymal fat pads to body weight increased with age. The weight of extensor digitorium longus, coleus and plantaris increased from 1 to 4 months, but it decreased at 6, 12 months. The protein content of muscles decreased or increased through- out 12 months. The T cell proliferation response to Con A stimulation was significantly lower at 6 months than 1 month and lower at 12 months than 6 months.