A good deal of attention has been directed of late to the alteration of the respiratory exchange and respiratory quotient during muscular exercise, with a view to elucidating the character of the metabolism and the behaviour of the respiratory centre. Hitherto, it has, as a rule, been the custom to make only a few determinations of the total respiratory exchange in any one experiment at rather long intervals from one another. Such a method, though it may give the general and broader features of the respiratory exchange, especially when experiments are multiplied, is clearly ill adapted to show any rapid variations that may occur. The individual periods during which the respiratory exchange is actually determined may be too long (this length is often essential, in order to render negligible slight errors which would become significant if it were curtailed), and the long intervals between the different determinations are undesirable. One of us, in conjunction with Haldane, Henderson, and Schneider, attempted to obtain information on the course of the total respiratory exchange in the period of rest immediately following a short and violent muscular exertion at an altitude of over 14,000 feet on Pike’s Peak, using for this purpose the bag method of Douglas. On this occasion, four determinations of the total respiratory exchange were made in each experiment at different intervals after the cessation of the muscular exertion, and, by making a considerable number of experiments, it was possible to obtain a fairly complete picture of the course of events in the hour-and-a-half immediately succeeding the muscular exertion. The main disadvantage was that, as the experiments had to be made on different days, the initial values for the resting respiratory exchange and respiratory quotient varied considerably in the different experiments. From a consideration of these experiments, it was, however, evident to us that the bag method could easily be adapted to give a practically continuous record of the respiratory exchange in a single experiment, and that the result would be infinitely more satisfactory than that obtained from a few observations made in each of a number of different experiments.