The aim of this study was to assess the effects of acute passive stretching on cycling efficiency during an exercise of heavy intensity. After maximum aerobic power (V̇O2max) assessment, nine active males (24 ± 5 years; stature 1.71 ± 0.09 m; body mass 69 ± 7 kg; mean ± standard deviation) performed tests at 85% of V̇O2max (Ẇ85) until exhaustion, with and without pre-exercise stretching. During the tests, we determined the gas exchange, metabolic and cardiorespiratory parameters. With stretching, no differences in V̇O2max occurred (3.64 ± 0.14 vs 3.66 ± 0.07 L/min for stretching and control, respectively). During Ẇ85, pre-exercise stretching (i) decreased time to exhaustion (tlim) by 26% (P<0.05); (ii) increased average V̇O2 by 4% (3.24 ± 0.07 and 3.12 ± 0.07 L/min in stretching and control, respectively; P<0.05); and (iii) reduced net mechanical efficiency (enet) by 4% (0.185 ± 0.006 and 0.193 ± 0.006 in stretching and control, respectively; P<0.05). Although acute passive stretching did not have an effect on V̇O2max, tlim and enet during heavy constant load exercise were significantly affected. These results are suggestive of an impairment in cycling efficiency due to changes in muscle neural activation and viscoelastic characteristics induced by stretching.