The physiological responses of high (HF) and low fit (LF) individuals at given perceived exercise intensities were compared to ranges provided by the American College of Sports Medicine (ACSM). Participants were 7 LF and 8 HF men between the ages of 22 and 26 years. All participants performed a maximum oxygen uptake and lactate threshold test and two 15-min experimental runs in which they exercised at a constant perceived exercise intensity (RPE 13 and 17). The LF group exhibited significantly greater maximum oxygen uptake reserve (%VO[subscript 2]R; p less than 0.05) and velocity of lactate threshold (p less than 0.01) values than HF at RPE 13 and 17. Both groups had significantly greater %VO[subscript 2]R and maximum heart rate values at RPE 13 in comparison with the ACSM ranges, using the highest value for the given range (p less than or equal to 0.001). (Contains 3 tables and 2 figures.)