.Rats and mice were kept in a decompression chamber at 52 kPa (390 mmHg) for 1-4 weeks and their hearts and lungs were compared with littermate control animals. In both species growth was retarded in the hypoxic environment.2. In both species small peripheral lung vessels became thickened, developing two elastic laminae with a muscular coat between. A method was developed for assessing these changes in large numbers of animals. The number of thick-walled vessels was still high after 4 weeks' recovery in a normal environment. Pulmonary vascular resistance, measured by a perfusion method, increased in animals kept in the decompression chamber.3. Mouse lungs became heavier than controls; the increase was not due to a greater fluid content. Rat lungs were heavy in relation to body weight but not heavier than controls; there may have been slight thickening of alveolar walls. Chest areas, measured from radiographs, were large relative to body weight in hypoxic rats. 4. The relationship between right and left ventricular weight and body weight was studied in normal rats and mice. The left ventricle grew about four times more quickly than the right. Changes in ventricular weights during exposure in the decompression chamber and subsequent recovery in a normal environment were related to these normal growth curves. 5. In both species the right ventricle grew abnormally fast in the decompression chamber. It was absolutely heavier than that of controls and relative to body weight was extremely heavy. After 4 weeks' recovery the relationship between right ventricular weight and body weight was nearly normal; this was achieved by retarded growth or actual loss of weight.6. In mice the left ventricle grew normally in the decompression chamber and was heavy in relation to body weight. In rats its growth was retarded in the chamber and was normal in relation to body weight.