The contribution of respiratory muscle O2 uptake ((.)V(O2RM)) to the development of the slow component of O2 uptake kinetics ((.)V(O2SC)) is unclear. The aim of the present study was to examine the impact of respiratory muscle unloading (via breathing, a He-O2 mixture) on the amplitude of (.)V(O2SC) during exercise performed below (B-RCT) and above the respiratory compensation threshold (A-RCT). We hypothesized that breathing He-O2 would reduce the amplitude of the (.)V(O2SC) by a greater amount during exercise performed A-RCT than B-RCT. Eight healthy male recreational cyclists performed constant load cycling in four sets of conditions: (1) B-RCT breathing normal air; (2) B-RCT breathing He-O2; (3) A-RCT breathing normal air; and (4) A-RCT breathing He-O2. Breathing He-O2 did not significantly attenuate the (.)V(O2SC) during exercise performed B-RCT (-3+/-14%, P >0.05). However, breathing He-O2 significantly reduced the (.)V(O2SC) during exercise A-RCT(-45+/-6%, P <0.05). The attenuated (.)V(O2SC) while breathing He-O2 is likely to reflect a decreased (.)V(O2RM). Minute ventilation was not +/-different between normal air and He-O2 breathing trials either B-RCT or A-RCT. However, operating lung volume was significantly lower when breathing He-O2 during exercise performed A-RCT (-12+/-3%, P <0.05). These findings suggest that (.)V(O2RM) comprises a greater proportion of the (.)V(O2SC) when exercise is performed A-RCT compared with B-RCT. Therefore, the impact of breathing He-O2 was more pronounced during exercise A-RCT. Furthermore, changes in operating lung volume and the work of breathing appear to play an important role in the development of the (.)V(O2SC).