1. Responsiveness to CO2 was measured in forty patients with chronic airways obstruction in terms of ventilation and rate of isometric inspiratory pressure change [(dP/dt)max.].
2. The ventilatory response was below the normal range in eighteen out of twenty-two patients with normal arterial CO2 tensions and in all of eighteen patients with CO2 retention.
3. The (dP/dt)max. response was distributed throughout the normal range in all but one of the patients with normal arterial CO2 tension. In all the patients with CO2 retention the (dP/dt)max. response was either at or below the lower limit of the normal range.
4. Although the ventilatory responses correlated significantly with FEV1 there was no such correlation for the (dP/dt)max. responses.
5. The (dP/dt)max. response showed a significant negative correlation with Pa,co2.
6. It is believed that the (dP/dt)max. response to CO2 can be used to assess central CO2 responsiveness in subjects with airways obstruction independently of mechanical factors limiting their ventilation.