Imperceptible levels of proportional assist ventilation applied throughout inspiration reduced inspiratory time (T_I) in awake humans. More recently, the reduction in _I was associated with flow assist, but flow assist also reaches a maximum value early during inspiration. To test the separate effects of flow assist and timing of assist, we applied a pseudorandom binary sequence of flow assisted breaths during early, late, or throughout inspiration in 8 normal subjects. We hypothesized that imperceptible flow assist would shorten _I most effectively when applied during early inspiration. Tidal volume, integrated respiratory muscle pressure per breath, _I, and expiratory time were recorded. All stimuli (early, late, or flow assist applied throughout inspiration) resulted in a significant increase in inspiratory flow; however, only when the flow assist was applied during early inspiration was there a significant reduction in _I and in the integrated respiratory muscle pressure per breath. These results provide further evidence that vagal feedback modulates breathing on a breath-by-breath basis in conscious humans within a physiological range of breath sizes.