Coronal slices of the rat medulla were used for an electrophysiological study of the nucleus of the tractus solitarius isolated in vitro. Half of the spontaneously active neurons in the ventral NTS exhibited a rhythmic repetitive spike discharge. In contrast, bursting activity was virtually absent in the isolated NTS. Resetting of repetitive discharges could be obtained by electrical stimulation of the glossopharyngeal-vagal afferent terminals in the tractus solitarius. These results are discussed in relation to the proposed role of the NTS in the neurogenesis of the respiratory rhythm.