Humoral factors play an important role in the control of exercise hyperpnea. The role of neuromechanical ventilatory factors, however, is still being investigated. We tested the hypothesis that the afferents of the thoraco-pulmonary system, and consequently of the neuromechanical ventilatory loop, have an influence on the kinetics of oxygen consumption (VO ₂), carbon dioxide output (VCO₂), and ventilation (VE) during moderate intensity exercise. We did this by comparing the ventilatory time constants of exercise with and without an inspiratory load. Fourteen healthy trained men (age 22.6 ± 3.2 years) performed a continuous incremental cycle exercise test to determine maximal oxygen uptake (VO ₂ max = 55.2 ± 5.8 ml.min^-1.kg^-1). On another day, after unloaded warm-up they performed randomized constant-load tests at 40% of their VO ₂ max for 8 minutes, one with and the other without an inspiratory threshold load of 15 cm H₂O. Ventilatory variables were obtained breath by breath. Phase II ventilatory kinetics (VO ₂, VCO₂ and VE) could be described in all cases by a mono-exponential function. The bootstrap method revealed small coefficients of variation for the model parameters, indicating an accurate determination for all parameters. A paired t-tests showed that the addition of the inspiratory resistance significantly increased the time constants () during phase II of VO ₂ (43.1 ± 8.6s vs. 60.9 ± 14.1s; P<0.001), VCO₂ (60.3 ± 17.6s vs. 84.5 ± 18.1s; P<0.001) and VE (59.4 ± 16.1s vs. 85.9 ± 17.1s; P<0.001). The average rise in the time constants was 41.3%for VO ₂, 40.1% for VCO₂ and 44.6% for VE. CONCLUSION: The time constant changes indicated that neuromechanical ventilatory factors play a role in the ventilatory response to moderate exercise.