HORSTMAN, DONALD H., RICHARD WEISKOPF, and SUMNER ROBINSON. The nature of the perception of effort at sea level and high altitude. Med. Sci. Sports. Vol. 11, No. 2, pp. 150-154, 1979. The purpose of this study was to compare perceptual responses (RPE) and selected physiological measures during both short term and prolonged exercise of equal relative intensities at 4300 m to those at sea level. Specifically, we compared results obtained (n = 20) for 6 min of exercise at 60, 80 and 95% Vo2max and at 5 min intervals during exercise to exhaustion at 85% Vo2max. At 4300 m, Vo2max was reduced 19%, while VEmax and Rmax increased 17 and 8%, respectively; HRmax and RPEmax was unchanged. For any given relative exercise intensity, Vo2 and absolute exercise intensity (kpm • min−1) were reduced, while VE 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, we 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 tachypnea and tachycardia are of sufficient magnitude to be perceived as extremely stressful.