Endurance training reduces the ventilatory response to a given level of work, and there is evidence that endurance athletes possess attenuated chemosensitivity at rest; but it is unclear whether attenuation persists during exercise. We compared the carbon dioxide sensitivity (S) of endurance-trained (ETG), sprint-trained (STG), and control subjects (CG), at rest and during cycle ergometry. Steady-state carbon dioxide (CO2) inhalation was employed; ventilatory parameters were measured using an ultrasonic flowmeter linked to a computer. CO2 concentrations were measured at the mouth using an infrared CO2 analyzer or mass spectrometer. Mean resting CO2 sensitivity of the ETG was significantly lower than that of the STG (P < 0.05), but not the CG (P < 0.058). S increased from rest to exercise in all endurance-trained subjects, but the responses of the STG and CG were varied. Compared to rest, mean S was significantly higher during exercise for the ETG, but not for the STG or CG. S was the same in all groups during exercise. During air breathing exercise all subjects were mildly hypercapnic. The ETG showed the greatest rise in mean alveolar PCO2, but this could not be attributed to attenuated chemosensitivity since responsiveness during exercise was identical in all three groups.