The quantity of the alimentary fill, expressed as a percentage of the live weight, has been determined with ten groups of albino rats, one group being fasted, the other groups being given different dietary treatments. The ratio of fill to gross body weight was found to be somewhat affected by both the kind and amount of feed, such effects being more prominent at the maintenance level than at higher planes of nutrition. The results indicate that with increase in food there is a proportionately greater increase in the rate of passage of food residues through the alimentary tract. The alimentary fill of rats which received food was greater than that of fasting rats by 2.4 to 8.2 per cent of the gross body weight, in accord with the dietary treatment. It is therefore important to consider alimentary fill when comparing the metabolism of animals subjected to different dietary treatments.