Tests in which a group of thirty-three rats were allowed the same food ad libitum and groups of thirty-seven, thirty-seven and thirty rats were fasted 1 day in 4, 3 and 2, respectively, after the age of 42 days, showed that the apparent life span was increased by the intermittent fasting. The optimum amount of fasting appeared to be fasting 1 day in 3 and this increased the life span of littermate males about 20% and littermate females about 15%. However, the pre-experimental condition of the individual rats was also found to be an important factor determining the life spans. No drastic retardation of growth was produced by the intermittent fasting but the development of mammary tumors was retarded in proportion to the amount of fasting.