Changes in expiratory time in conscious sham-operated rats (SHAM) and in rats with bilateral carotid sinus nerve transection (CSNX) following bolus injection of morphine (10 mg/kg. i.v.), and then exposure to hypoxic challenge (10% O2, 90% N2) for 20 min (top panel) or hypercapnic challenge (5% O2, 21% O2, 74% N2) for 20 min (middle panel) and then re-exposure to room-air. The maximal and total changes in expiratory time (Te) are shown in the bottom left and right panels, respectively. The data are presented as mean ± SEM. There were 6 rats in each group.