IN the present paper we propose to describe the changes in ventilation, blood flow, pulse rate, respiratory exchange and alveolar CO2 tension taking place in man during the first few minutes of light or heavy work and we shall attempt further an inquiry into the mechanism of these changes. The experinments have been made by means of the bicycle ergometer and respiration apparatus described in another paper' in which also the main parts of the technique employed have been set forth in detail. We have made experiments on six different suibjects. Three of these, J. L., F. N. and J. J., are trained, mnore or less, to sudden and violent exertions, while the other three, though able to do a fair amount of work over a long period, are not trained to sudden exertions. When told to start with a heavy load the three first named subjects will begin at a very rapid rate, which soon declines, while the other threeeven when told to start as rapidly as possible, and meaning to do sowill begin comparatively slowly and work up during a certain time. (Compare for example Exps. X and IX, p. 135.) On the subjects J. J., J. K. and H. P. we have made determinations of pulmonary volumes, ventilations and rates of work only, under varying conditions. On A. K. and F. N. we have made a few " complete " experiments involving determinations of the respiratory exchange and the composition of the alveolar air at several moments, especially during the first minute of work. The most detailed study has been made on J. L. The respiration of this subject is deep, slow and extremely regular, which greatly facilitates the correct sampling of the alveolar air. We wish to state explicitly-though it may seem superfluous-that the breathing in all cases (except when a subject was told to breathe