The aims of noninvasive ventilation include the correction of hypoventilation and unloading of inspiratory muscles. Volume cycled flow generators, bi-level positive airway pressure and continuous positive airway pressure techniques have all been used with face and nasal masks. We have compared these modes of ventilatory support, administered by a nasal mask in stable, awake outpatients with chronic obstructive pulmonary disease or neuromusculo-skeletal disease in respect of their effects on ventilation, inspiratory muscle effort and oxygen saturation. There were no clinically significant differences between the volume cycled flow generator and bi-level positive airway pressure methods; compared to spontaneous ventilation, oxygen saturation increased and inspiratory muscle effort decreased. Tidal volume increased and respiratory rate reduced, the largest changes occurring with bi-level positive airway pressure. Only the volume cycled flow generator increased minute ventilation significantly. Ventilation and inspiratory muscle effort were unaffected by continuous positive airway pressure but oxygen saturation was lower than during spontaneous ventilation. In awake, stable outpatients acclimatised to nasal ventilation there were no clinically significant differences between volume cycled flow generator and bi-level positive airway pressure techniques, but continuous positive airway pressure was less effective.