Introduction .—It has long been known that the dyspnœa produced by strenuous exercise, such as running or rowing, disappears if the work is continued and is replaced by a sense of great relief, the so-called “second wind.” A certain time is necessary for the adjustment or accommodation which produces this sensation; short-distance runners do not experience it, but those who are accustomed to long runs over the same course can predict at which lap or point they will obtain the relief of second wind. There are also individual differences; in some men the sensation is very definite, in others so indefinite that it is unrecognised. The observations of Cook and Pembrey showed that during dyspnœa the percentage of carbon dioxide in the alveolar air was raised above the resting value and the respiratory quotient was unity or above unity, but daring the hyperpnœa following the onset of second wind the percentage of carbon dioxide and the respiratory quotient fell; the amount of air breathed per minute was less during hyperpnœa than during dyspnœa; the rectal temperature showed a rise of about 1° F. during second wind, and as a rule there was a close association between the onset of sweating and second wind. Carbon dioxide appeared to be the chief factor in the adjustment of the respiratory and circulatory systems to the demands of the muscles for an adequate supply of blood.