MR. ODELL'S remarks,1 regarding Dr. Argyll Campbell's experiments, which were designed to test the power of acclimatisation of animals to low oxygen pressure, are not quite accurate. These experiments were undertaken to supply information that was wanting, and to endeavour to explain the adverse symptoms recorded in the official reports of the Everest expeditions. The animals were exposed to gradual decrease of oxygen pressure such as occurs in climbing. They showed all the well-known signs of so-called acclimatisation, for example, increase of red blood cells, increased depth of breathing, etc. Some of the experiments, unlike the expeditions, did not stop at a pressure equivalent to 28,000 ft. but went on to one near 30,000 ft. Some of the animals were exposed to 7–7½ per cent of an atmosphere of oxygen for eight days continuously, and recovered completely when put under normal oxygen pressure again. In spite of heroic efforts, no man has been exposed to so low a pressure for even one day, and Dr. Campbell's animals hold the record for toleration of low oxygen pressure by mammals.