A B S T R A C T The respiratory responses to hypercapnia alone and to hypercapnia and flow-resistive loading during inspiration were studied in normal individuals and in eucapnic and hypercapnic patients with chronic airways obstruction. Responses were assessed in terms of minute ventilation and occlusion pressure (mouth pressure during airway occlusion 100 ms after the onset of inspiration).Ventilatory responses to CO2 (AV/APco2) were distinctly subnormal in both grouips of patients with airways obstruction. The two groups of patients, however, showed different occlusion pressure responses to CO2 (AP100/APCo2): AP100/APCo2 was normal in the eucapnic patients but subnormal in the hypercapnic patients. Flow-resistive loading during inspiration reduced AV/APCo2 both in normal subjects and in patients with airways obstruction. The occlusion pressure response to CO2 increased in normal subjects during flow-resistive loading but remained unchanged in both grouips of patients with chronic airways obstruction.These results indicate that while chemosensitivity as determined by AP100dAPCo2 is impaired only in hypercapnic patients with chronic airways obstruction, an acute increase in flow resistance elicits a subnormal increase in respiratory efferent activity in both euicapnic and hypercapnic patients.