To determine the relation of retarded growth to aging, Dr. Barnes, Miss Sperling, and Dr. Mccay followed the growth of rats for 1200 days and observed the rate of growth of the tibia, the appearance of the epiphyseal line in the tibia, the ratio of the body weight to the tibial length, and the ratio of body length to tibial length. The rats were divided into four groups: (1) control, (2) retarded by restricted diet, (3) retarded 330 days and then realimented, and (4) retarded 900 days and then realimented. The tibia of no retarded growth rat grew to a final length as great as the average final tibial length of the controls.