The purpose of this study was to describe the pattern of change in effort sense and the value of this pattern in predicting work end-point at relatively high work intensity (80% VO2 max). The patterns of change of various physiological functions were also observed. Two modes of work (walking and running) were compared to ascertain generalizability of results. 26 healthy male volunteers served as subjects. Time to exhaustion (ET) did not differ between walking and running. As work continued during both tasks, significant increases of VE, VE/VO2, VE/VCO2 and HR and a significant decrease of ETCO2 were observed; while VO2 and R remained fairly constant. VO2 and VE during the run were about 5% greater than during the walk; there were no differences in other measures. Ratings of perceived exertion (RPE) from the Borg Scale were identical for both conditions, increasing in a near linear fashion from a value of 12.9 at 25% of total work time to 18.9 at exhaustion. Ratings obtained at 25 and 50% ET were extrapolated to time of exhaustion; the point of intercept corresponded to ratings of perceived exertion for maximal work. At exhaustion, subjects rated perception of respiratory exertion for the walk as less than that for the run; perception of leg exertion was not different for the two conditions. Plasma lactate, epinephrine and norepinephrine concentrations following exercise did not differ between the two conditions. The findings for the walking experiment were essentially replicated in a second investigation involving another 28 subjects. It is concluded that, with the exception of VO2 and some ventilatory parameters, walking and running at the same relative work intensity resulted in comparable perceptual and physiological responses. Psychophysical judgments made early during work were reasonably accurate predictors of exhaustion time.