The purpose of the present study was to characterize respiratory-related neurons within the nucleus raphe obscurus (NRO) and to examine their responses to hypercapnia and vagal afferent stimulation. Cat was decerebrated, paralyzed, vagotomized and ventilated. End-tidal fractional concentration of CO2 was maintained at hyperoxic normocapnia. The phrenic nerve was isolated and its activity was monitored. Neuron activity within the NRO was recorded and evaluated to the response to hypercapnia or vagal afferent excitation. Sixty-six of 133 neurons recorded showed respiratory-related activities. They were classified as expiratory-inspiratory (EI), inspiratory-expiratory (IE), tonic inspiratory-expiratory (T-IE) and tonic expiratory-inspiratory (T-EI) or expiratory (E) patterns according to their time-related with phrenic discharge. Responses of 40 respiratory-related neurons to hypercapnia were examined. Discharge rate of 18 neurons was significantly reduced in response to hypercapnia while 8 neurons was enhanced. The remaindering neurons displayed irregular pattern as the concentration of carbon dioxide was increased. With vagal stimulation, most of the neurons (9 of 13) showed a reduction of discharge rate (p＜0.05) whereas the others were either increased or unaffected. These results indicate that respiratory-related neurons within the NRO can be modulated by the inputs from the chemoreceptors and vagal afferents. It also suggest that the NRO may play a role in the regulation of respiration.