1. Weanling (24-d-old) male rats were maintained at their body-weight for 1 month by restricting the intake of their normal diet. The animals were then rehabilitated for 0, 3, 7, 10 or 16 d. Control animals were given an unrestricted diet and some killed at the same body-weight as the experimental animals and others at the same age.
2. The forebrain, cerebellum, brain stem, heart, lungs, liver, spleen, kidneys, testes, and three sections of the alimentary tract were weighed, and DNA, RNA and protein contents were determined in the brain parts and liver.
3. During rehabilitation the weight of the body, corrected for the weight of the gut contents, showed a rapid gain between 7 and 10 d, which was synchronous with a rapid gain in weight of the testes.
4. The weight of the majority of organs relative to body-weight was maintained during both undernutrition and rehabilitation, the most marked exceptions being the stomach, which increased in relative weight during undernutrition, and maintained a high relative weight during rehabilitation, and the spleen, which lost weight during undernutrition, and on rehabilitation gained weight very rapidly to achieve a high relative weight.
5. The weight of the forebrain fell during undernutrition, due to a loss of water, and the weight of the brain stem rose. In the forebrain, DNA and the protein: DNA ratio were unchanged throughout, whereas a marked loss of RNA occurred during undernutrition, which was restored during rehabilitation.
6. The weight of the liver remained unchanged during undernutrition, despite increases in the amounts of DNA and protein. The amount of liver RNA decreased during undernutrition, but on rehabilitation showed an immediate and rapid increase. The variables measured in the liver were normal relative to body-weight, within 10 d of rehabilitation.
7. It is suggested that the growth occurring on rehabilitation is a balanced response to a single stimulus, partly mediated at the cellular level by RNA.