In the experiments described below an endeavour has been made ( a ) to follow the course of recovery from vigorous exercise of short duration, ( b ) to determine whether it be possible to arrive at some average figure which adequately represents the degree of recovery of different subjects at any time after the end of exercise, and ( c ) to investigate the magnitude and direction of any alteration that may occur in the basal metabolism as a result of the muscular work performed. Attention was confined to exercise such as described above, as sufficient data are available in the previous work of A. V. Hill for following the course of the recovery process after prolonged exercise of a similar nature. Procedure . The object of the experiments was to determine the total amount of oxygen used during successive time-periods from the end of exercise, i . e ., to follow the rate at which the oxygen debt incurred in the exercise was “ paid off ” during the recovery period. Procedure . The object of the experiments was to determine the total amount of oxygen used during successive time-periods from the end of exercise, i. e ., to follow the rate at which the oxygen debt incurred in the exercise was “paid off” during the recovery period.