The purpose of the present investigation was to examine the relationship between the percent heart rate reserve (%HRR) in arm exercise and the corresponding percent oxygen uptake (VO2) reserve, and to compare this relationship to that occurring in running. Fourteen male physical education students took part in the study. Each subject performed a maximal running exercise test and a maximal arm cycling test. The subjects also performed three submaximal exercise bouts (in both exercise modes) at 30%, 60% and 80% of their HRR. The subjects were monitored for their heart rate (HR) at rest, maximal HR (HRmax), HR at submaximal work loads. maximal VO2 (VO2max), VO2 at rest and VO2 at submaximal loads. For each subject, load and exercise mode, %HRR and %VO2 reserve were calculated (from HRmax and VO2max as measured during running and arm cycling) and the relationship between the two was evaluated. The main finding of the present investigation is that the prediction of %VO2 reserve in arm cycling from %HRR is grossly overestimated when calculated from HRmax and VO2max measured during running. The prediction is better but still overestimated when calculated from HRmax and VO2max measured during arm cycling. The findings indicate a better prediction of %VO2 reserve from %HRR for running than for arm exercise. These findings should be taken into consideration when prescribing the target HR for arm training.