Extracellular recordings were made from 123 synaptically activated neurones in the nucleus tractus solitarius of the chloralose-anaesthetized cat. 2. Ninety-one neurones were activated by electrical stimulation of cardiac or pulmonary vagal branches and thirty-two by stimulation of the aortic nerve. 3. Ionophoretic application of GABA abolished or markedly reduced the evoked or spontaneous activity of each neurone tested. These inhibitory effects were antagonized by the simultaneous ionophoretic application of bicuculline. 4. Glycine inhibited the evoked activity of 60% of neurones tested. This inhibition could be antagonized by the simultaneous application of strychnine. 5. Application of bicuculline alone increased the evoked or spontaneous activity of a large proportion of the neurones; strychnine alone had no significant effect on the evoked or spontaneous activity of the neurones. 6. These results are consistent with the hypothesis that GABA acts as an inhibitory transmitter substance within the nucleus tractus solitarius.
