It is widely accepted that the relationship between oxygen consumption and body weight obtained during exercise on a bicycle ergometer differs from that obtained during treadmill walking. Experimental evidence to support this claim is lacking. To examine this difference a group of subjects (body weight 41—81 kg) undertook a predetermined level of submaximal exercise on a bicycle ergometer and on a treadmill. Oxygen consumption was measured in a steady state at rest (i.e. sitting on the bicycle ergometer and standing on the treadmill) and during the two modes of exercise. A significant positive correlation between oxygen consumption and body weight was obtained under all four conditions of measurement. At rest the two regression lines did not differ in slope or elevation. During exercise the slope and the elevation of the line obtained from treadmill walking were significantly greater than from bicycle ergometer exercise. The ‘metabolic cost’ of bicycle ergometer exercise, (Vo2 during exercise—Vo2 at rest), showed no significant correlation with body weight. In contrast, there was a significant positive correlation during walking. It is suggested that these differences have arisen due to a different proportion of the total body weight supported by the subject in the two forms of exercise.