Perceptual responses (RPE) and selected physiological measures were compared in humans during short term and prolonged exercise of equal relative intensities at 4300 m to those at sea level. Results obtained (20) for 6 min of exercise at 60, 80, and 95% .ovrhdot.VO2 max [maximum O2 consumption] and at 5 min intervals during exercise to exhaustion at 85% .ovrhdot.VO2 max were compared. At 4300 m, .ovrhdot.VO2 max was reduced 19%, while .ovrhdot.E max [maximum volume of expired gas] and Rmax [maximum heart rate] increased 17 and 8%, respectively; HRmax [maximum respiration] were unchanged. For any given relative exercise intensity, .ovrhdot.VO2 and absolute exercise intensity kilopound meter/min were reduced, while .ovrhdot.E was about 12% and R about 7% greater at 4300 m; HR was unchanged. At 4300 m, RPE at the lower intensities of submaximal exercise and early during prolonged exercise were significantly less than at sea level. These differences were reduced and finally eliminated as exercise intensity increased toward maximal or as prolonged exercise continued to exhaustion. Endurance time to exhaustion at 4300 m was not different from that at sea level. To account for the perceptual differences between exercise at 4300 m and sea level, it was proposed that local factors (muscular strain) exert greater influence on the perception of effort at exercise intensities which do not greatly stress ventilation and circulation, while central factors exert greater influence on the perception of effort at exercise intensities at which tachycardia are of sufficient magnitude to be perceived as extremely stressful.