The primary purpose of this investigation was to determine if a safe and effective physical conditioning heart rate (HR) could prescribed by perception of exertion. Ratings of perceived exertion (RPE) were requested from 10 normal adult men during treadmill exercise trials at 4.7, 6.5, 9.7, 11.3, and 12.9km/hr (T1). Subjects were then requested to subjectively regulate their own treadmill speed during 2 separate trials (T2 and T3) at the RPE reported for each speed during T1. Speed and HR at equivalent RPE were compared during T1, T2, and T3. Regression analyses revealed that there was no difference in speed across all RPE between the 3 trials; however, HR was seen to become progressively higher during T2 and T3 than during T1 as speed and RPE decreased. HR reliability was significant (p < 0.05) during running but not significant (p > 0.05) during walking. It is concluded that prescription of exercise by RPE can produce safe, effective, and reliable conditioning HR above 150bpm (80% HRmax) and running speeds above 9km/hr. Use of RPE for exercise prescription below these levels can result in inaccurate and unreliable conditioning heart rates. This method of exercise prescription has limitations and could result in dangerously high HR is used in cardiac rehabilitation programs, in which strict adherence to target HR is essential.