Ambulatory heart rate monitors and clinical electrocardiographic (ECG) devices are capable of measuring the length of consecutive cardiac periods (RR intervals). The aim of the study was to assess the agreement between the Polar 810s heart rate monitor (Polar) and the Reynolds digital ambulatory ECG using Pathfinder software version 8.4 (Reynolds v8.4) during cycle ergometry. For this purpose, eight subjects completed incremental cycling exercise that began at 60 W and increased by 30 W each 2-minute period until volitional fatigue. Simultaneous recording of the ECG (Reynolds Pathfinder), RR interval (Polar), and respiratory parameters (Metamax 3B) were undertaken at rest and throughout the exercise period. No significant differences were found in RR intervals measured by Polar and Reynolds v8.4 at any relative intensity. Polar and Reynolds v8.4 displayed strong linear relationships at all relative intensities (r2=0.927 to 0.998). Bland and Altman analyses between Polar and Reynolds v8.4 consistently demonstrated minimal bias in absolute RR interval (<0.10 ms) and the limits of agreement for group differences in RR interval and heart rate were less than +/- 10 ms and +/- 2 beats.min (-1) for all relative intensities, respectively. Power spectral analysis provided similar results for both systems in all bandwidths studied during rest and low intensity exercise. However, significant differences and large relative limits of agreement (>100 % of mean of paired means) were identified in UF at intensities > 40 % VO2max, HF at intensities > 60 % VO2max and LF during exercise at 80-100 % VO2max. These findings demonstrate that RR intervals and heart rate measurements obtained using Polar and Reynolds v8.4 are in good agreement. However, caution should be exercised when interpreting spectral analysis of RR interval data derived from different acquisition systems during physical activity.