The reflex mechanism of the short-latency inhibitory reflex to transient loading of human inspiratory muscles is unresolved. Muscle afferents mediate this reflex, but they may act via pontomedullary inspiratory centers, other bulbar networks, or spinal circuits. We hypothesized that altered chemical drive to breathe would alter the initial inhibitory reflex if the neural pathways involve inspiratory medullary centers. Inspiration was transiently loaded in 11 subjects during spontaneous hypercapnic hyperpnea and matched voluntary hyperventilation. Electromyographic activity was recorded bilaterally from scalene muscles with surface electrodes. The latencies of the initial inhibitory response (IR) onset (32 +/- 0.7 and 38 +/- 1 ms for spontaneous and voluntary conditions respectively, P < 0.001) and subsequent excitatory response (ER) onset (80 +/- 2.9 and 78 +/- 2.6 ms, respectively, P = 0.46) and the normalized sizes of IR (65 +/- 2 and 67 +/- 3%, respectively, P = 0.50) and ER (51 +/- 8 and 69 +/- 6%, respectively, P = 0.005) were measured. Mean end-tidal Pco(2) was 43 +/- 1.5 Torr with dead space ventilation and was 14 +/- 0.6 Torr with matched voluntary hyperventilation (P < 0.001). A mean minute volume >30 liters was achieved in both conditions. The absence of significant difference in the size of the IR suggested that the IR reflex arc does not transit the brain stem inspiratory centers and that the reflex may be integrated at a spinal level. In voluntary hyperventilation, an initial excitation occurred more frequently and, consequently, the IR onset latency was significantly longer. The size of the later ER was also greater during voluntary hyperventilation, which is consistent with it being mediated via longer, presumably cortical, pathways, which are influenced by voluntary drive.