1. A technique has been developed for assessing CO2 responsiveness by measuring the maximum rate of isometric inspiratory pressure change at the mouth [(dP/dt)max.]. 2. By use of a rebreathing technique, the (dP/dt)max. response to CO2 was shown to correlate well the ventilatory response in thirty-two normal subjects. 3. The addition of an external flow resistance sufficient to reduce the ventilatory response by a mean of 33.4% produced no significant mean change in the (dP/dt)max. response in thirty subjects. 4. In six patients recovering from bronchial asthma, reduction of airways obstruction led to a mean increase in the ventilatory response of 109% without any significant mean change in the (dP/dt)max. response. 5. An increase in lung volume did not reduce the (dP/dt)max. response in five normal subjects. 6. At very high lung volumes, six normal subjects were able to develop a higher (dP/dt)max. during voluntary inspiratory efforts than has been recorded during spontaneous breathing response to CO2. 7. It is believed that (dP/dt)max. represents the initial rate of development of force by the inspiratory muscles before this can be modified by mechanical loading, proprioceptive feedback mechanisms or conscious response and can therefore be used to study changes in the motor output of the respiratory centre in response to ventilatory stimuli independently of pulmonary mechanics.