Abstract A stereotyped series of postsynaptic potentials produced in cat hypoglossal motoneurons by stimulation of the cerebral cortex, the inferior alveolar nerve or the lingual nerve was studied. These include an excitatory postsynaptic potentials (EPSP) and subsequently 3 different types of inhibitory postsynaptic potential (IPSPs). The first is a short-lasting IPSP which was blocked by strychnine administration. The second is a γ-aminobutyric acid (GABA) IPSP which was blocked by picrotoxin administration. This IPSP was sensitive to membrane polarization and dependent on a conductance increase. The third is a long-duration hyperpolarizing potential which was enhanced by the injection of picrotoxin and insensitive to membrane polarization. Moreover, we have demonstrated that the amplitude of cortically induced EPSPs decreased greatly with depolarization.