The primary objective of this study was to examine the relationship of the "fat burning" and aerobic zones. Subjects consisted of 36 relatively fit runners (20 male, 16 female) who completed a maximal exercise test to exhaustion on a motor-driven treadmill. The lower and upper limit of the "fat burning" zone was visually assessed by examining each individual graph. Maximal fat oxidation (MFO) was determined to be that point during the test at which fat metabolism in fat calories per minute peaked. The lower limit of the aerobic zone was assessed as 50% of heart rate reserve, whereas the upper limit was set at anaerobic threshold. Although the lower and upper limits of the "fat burning" zone (67.6-87.1% maximal heart rate) were significantly lower (p < 0.05) than their counterparts in the aerobic zone (58.9-76.2%), the considerable overlap of the 2 zones would indicate that training for fat oxidation and training for aerobic fitness are not mutually exclusive and may be accomplished with the same training program. Furthermore, it was determined that this training program could simultaneously meet the requirements of the American College of Sports Medicine for both aerobic fitness and weight control. Maximal fat oxidation occurred at 54.2% maximal oxygen uptake (VO2max). However, the great variability in response between individuals would preclude the prediction of both the "fat burning" zone and MFO, indicating a need for measurement in the laboratory. If laboratory testing is not possible, the practitioner or subject can be reasonably confident MFO lies between 60.2% and 80.0% of the maximal heart rate.