A comparison has been made between walking uphill (+ IV) and downhill (-TV) for 1 hr on a motor-driven treadmill on 7 healthy male subjects aged 21-41 years. Measurements included: oxygen intake (J"02)' and CO 2 output CVCOz)' minute ventilation (l71~). tidal volume (VT), mixed venous (PVC02) and alveolar (PAC02) Peo'}.. cardiac output (Q) and frequency (fn), tympanic temperature (TtvL sweat rate (8) and blood lactic acid (LA) concentration..For a given work the VO z cost of + 1fT was 5 times higher than the corresponding -IV, tho 'efficiency' values at the 20th min approaching +0·20 and -1·2 respectively. In -IV but not + IV there was a gradual decline ( . . . . . . . . . 20%) of efficiency with time. The VElVC02; VEfV T and PyCO,/ Vco, relationships were the same for both forms of exercise but PAC02 was lower in -lV. For a given V0 2 ,QandfH were found to be higher and arteria-venous O 2 difference lower in -lV but stroke volume was similar in both forms of work. ::1'ty and S were higher in -TV for given metabolic heat production but the lamer difference was resolved if S was rotated to total heat production. Equally the differences in LA production could be explained in terms of V0 2 • During -IV it is likely that the circulation to the working muscles is not limiting arid the greater physiological stress in this type of exercise is due to an increased amount of work dono on the subject which ultimately has to be dissipated as heat.

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