Purpose: Rating of perceived exertion (RPE) is a widely used tool to assess subjective perception of effort during exercise. The authors investigated between-subject variation and effect of exercise mode and sex on Borg RPE (6–20) in relation to heart rate (HR), oxygen uptake (VO₂), and capillary blood lactate concentrations. Methods: A total of 160 elite endurance athletes performed a submaximal and maximal test protocol either during cycling (n = 84, 37 women) or running (n = 76, 32 women). The submaximal test consisted of 4 to 7 progressive 5-minute steps within ∼50% to 85% of maximal VO₂. For each step, steady-state HR, VO₂, and capillary blood lactate concentrations were assessed and RPE reported. An incremental protocol to exhaustion was used to determine maximal VO₂ and peak HR to provide relative (%) HR and VO₂ values at submaximal work rates. Results: A strong relationship was found between RPE and %HR, %VO₂, and capillary blood lactate concentrations (r = .80–.82, all Ps < .05). The between-subject coefficient of variation (SD/mean) for %HR and %VO₂ decreased linearly with increased RPE, from ∼10% to 15% at RPE 8 to ∼5% at RPE 17. Compared with cycling, running induced a systematically higher %HR and %VO₂ (∼2% and 5%, respectively, P < .05) with these differences being greater at lower intensities (RPE < 13). At the same RPE, women showed a trivial, but significantly higher %HR and %VO₂ than men (<1%, P < .05). Conclusions: Among elite endurance athletes, exercise mode influenced RPE at a given %HR and %VO₂, with greater differences at lower exercise intensities. Athletes should manage different tools to evaluate training based on intensity and duration of workouts.