1. Weanling male rats were maintained at constant body-weight for 28 d by feeding them reduced amounts of their normal diet. They were rehabilitated for 0, 3, 7, 10 or 16 d, and compared with two sets of control groups, one set of the same body-weight and the other of the same age. 2. The quadriceps, gastrocnemius and anterior tibialis muscles from the left hind-limb were weighed, and DNA, RNA, extracellular protein and intracellular protein estimated in the quadriceps and gastrocnemius muscles. 3. Each muscle responded differently during undernutrition and rehabilitation if compared with "age controls", but if compared with "body-weight controls" there was a tendency for muscle weight to remain appropriate for body-weight. 4. The amount of DNA did not change in the gastrocnemius or quadriceps muscles during undernutrition, and on rehabilitation did not begin to increase until after 7 d. RNA decreased during undernutrition, but increased rapidly on rehabilitation. The rate of increase in RNA was greatest in the quadriceps during the first 3 d, but in the gastrocnemius was greatest between 3 and 7 d. The peak values for the rate of RNA increase corresponded with the initiation of intracellular protein accretion in each muscle. Extra cellular protein increased during undernutrition, and on rehabilitation, responded in the same way as DNA, that is, it increased only after 7 d. 5. It is concluded that the differences found between muscles of the hind-limb during rehabilitation are due mainly to differences in the rates of DNA and intracellular protein accretion. It is suggested that these differences are the result of a differential response of RNA to rehabilitation.