Perception of exertion (RPE) during exercise was studied in 8 lean and 8 obese young men. Each subject was tested twice to maximum (a) at 24°C on a bicycle ergometer, treadmill with and without excess weight (lean subjects were weighted to match the percent excess weight of obese subjects) and (b) at 32°C on the treadmill with and without excess weight. Oxygen intake (Vo2), ventilation (VE), heart rate (HR) and RPE were determined at each work load. No significant differences in RPE vs Vo2 were observed on the treadmill. Lean subjects reported higher RPE's on the bicycle than did obese subjects. All subjects rated the bicycle to be harder than the treadmill at the same Vo2. Excess weight did not affect RPE vs Vo2 in lean subjects. Obese subjects were not affected by the 32°C environment, while lean subjects rated higher at the same Vo2. RPE vs HR was unchanged under all conditions. Although there were differences between lean and obese subjects when RPE was related to absolute values of VE and Vo2, relative to maximal values there was no difference. Since the maximal HR was similar, there was no difference in RPE vs HR on an absolute or relative basis. RPE appears to be most closely related to the proportion of maximal capacity required for a given work load.