During postnatal maturation, development of respiratory activity and hypercapnic responses is controlled by a network of chemosensory cells that senses changes in CO2 and H+ ion concentration. The ventrolateral medulla is considered the primary site for central chemosensitivity, however, brain stem midline neurons have also been shown to contribute to ventilatory control of upper airway and chest wall muscle activities. In this study we sought to characterize the role of specific neurons within the midline brainstem at the so-called raphe obscurus on phrenic and hypoglossal nerve responses to hypercapnia during development. Studies were performed in ten decorticate, vagotomized, paralyzed and mechanically ventilated 14 to 20 day old piglets. Blockade of raphe obscurus neuronal activity induced by microinjections of lidocaine was associated with a significant reduction of phrenic and hypoglossal nerve responses to progressive hypercapnia. At any given end-tidal CO2, peak phrenic and hypoglossal nerve activities were significantly lower following blockade than in the control period. For example, at 7.5% CO2 phrenic activity (in arbitrary units) was 33.9±1.9 vs 18.3±4.1 (p<0.005, mean ± SEM) prior to vs following lidocaine blockade respectively. Corresponding hypoglossal activity was preferentially reduced to a greater degree than phrenic activity. for example, at 7.5% CO2 hypoglossal activity was 20.8±3.6 vs 5.6±2.1 (p<0.005) prior to vs following lidocaine blockade respectively. Injection of saline (control) solution at the same site had no effect. We conclude that impaired function of raphe obscurus midline neurons reduces respiratory responses to central chemosensory stimulation. We speculate that specific neuronal sites within the midline region of the brainstem are required for full expression of ventilatory responses to hypercapnia, and raise the possibility that alteration in their function may contribute to respiratory instability during early postnatal life.