We measured hypercapnic ventilatory responses using the rebreathing technique and ventilatory drive using mouth occlusion pressure in 15 normal subjects (6 with added external inspiratory resistance), 11 asthmatics, and 17 patients with chronic obstructive pulmonary disease (9 with chronic CO2 retention and 8 with normal values for arterial pco2). normal subjects, obstructed normal subjects, asthmatics, and patients with chronic obstructive pulmonary disease without CO2 retention had overlapping ventilatory responses. Ventilatory drive was increased in asthmatics and obstructed normal subject. Patients with chronic obstructive pulmonary disease without CO2 retention maintained a ventilatory drive similar to that of normal subjects, whereas patients with chronic obstructive pulmonary disease with chronic CO2 retention demonstrated blunted ventilatory drives as a group, even though 5 of 9 had normal drives. Patients with CO2 retention also had the greatest obstruction when compared to other groups. In some patients, chronic CO2 retention is primarily a consequence of mechanical end-organ limitation rather than a blunted neurorespiratory center output. Acute airway obstruction is associated with an increased drive, which may become reduced with chronic obstruction.