In three groups of subjects we studied heart rate (HR) and ventilatory responses to progressive eucapnic hypoxia, steady-state hypercapnia with and without hypoxia, and hyperoxic and hypoxic breathholding (BH). Groups were six subjects about 25 years after bilateral carotid body resection (BR), eight subjects of an equally long period after unilateral resection (UR), and three control subjects similar to the study groups in age and pulmonary function (C). During progressive hypoxia, HR increased more in BR than in UR and C subjects. Ventilatory response was lowest in BR subjects (as expected). Steady-state hypoxic hypercapnia (end-tidal PO2, 60 Torr) depressed HR significantly more in C than in BR and UR subjects. Again, ventilatory response was lower in BR than in C subjects. HR progressively increased during BH initiated in hyperoxia (end-tidal PO2, 200 Torr) and hypoxia (end-tidal PO2, 70 Torr). In the BR group, the HR increment during hypoxia was significantly larger than that during hyperoxia. No such difference was apparent in UR and C groups. Thus, hypoxia with or without hypercapnia tends to accelerate HR in BR subjects whereas either less tachycardia or slowing is seen in UR and C subjects.