When rats were undernourished so that they gained only 20 g. body weight between the 3rd and the 11th week of life:(1) The testes continued to enlarge; the seminiferous tubules developed, but the interstitial cells regressed.(2) The penis of some of the animals protruded; in others protrusion was first observed on the day after rehabilitation began.(3) The vaginae of more than half the females opened while the animals were still undernourished and had an abnormally small body weight.(4) The ovarian follicles appeared active, but no corpora lutea were found. When the rats were returned to unlimited food, body weight increased, corpora lutea were formed, but the testes remained large as a percentage of the weight of the body.