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 (V˙O2) 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 V˙O2 (V˙O2max), V˙O2 at rest and V˙O2 at submaximal loads. For each subject, load and exercise mode, %HRR and %V˙O2 reserve were calculated (from HRmax and V˙O2max 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 %V˙O2 reserve in arm cycling from %HRR is grossly overestimated when calculated from HRmax and V˙O2max measured during running. The prediction is better but still overestimated when calculated from HRmax and V˙O2max measured during arm cycling. The findings indicate a better prediction of %V˙O2 reserve from %HRR for running than for arm exercise. These findings should be taken into consideration when prescribing the target HR for arm training.