We have investigated the response of 12 normal, healthy subjects to resistance loading and ventilator assistance of spontaneous breathing. Three ventilators, the Hamilton Veolar, Engström Erica and Puritan Bennett 7200, were used to provide synchronized intermittent mandatory ventilation and two levels of pressure assistance. Total respiratory elastance and resistance were measured. The equivalent (negative) pressure of respiratory muscle activity (pmus) was then calculated from measurement of flow and pressure at the mouth. With ventilatory assistance, subjects maintained frequency, decreased inspiratory time and the magnitude of pmus, but increased tidal volume, thus not taking full advantage of ventilatory assistance. The waveform of pmus varied in detail within and between subjects and conditions, but the all-subject mean waveforms showed for all conditions a consistency of trajectory. Increasing the level of assistance decreased the duration and hence the (negative) peak value of pmus. The results suggest that some waveforms of flow or pressure from the ventilators may be more acceptable to patients than others, and that different patients may prefer different waveforms.