The responses of the unitary discharges of the vestibular nuclei to caloric stimulation of the middle ear cavity were investigated in pentobarbital-anesthetized cats and in the encephale isole preparations of unanesthetized cats. The frequency of spontaneous unitary discharges in the vestibular nuclei was significantly higher in unanesthetized cats than anesthetized ones. The responses of 32 vestibular unitary discharges to both cold and warm stimulation in the anesthetized animals were characterized into 3 types: the increased type of 10 units in which the frequency of the unitary discharges was increased by stimulation, the decreased type of 8 units in which the frequency was decreased by stimulation, and the refractory type of 14 units in which the frequency did not change by stimulation. In unanesthetized animals, the responses of 72 vestibular units were classified in the following 5 types: the increased type of 18 units, the decreased type of 6 units, the reversed type-I of 20 units in which the frequency of the unitary discharges was decreased by the cold stimulation and increased by warm the reversed type-II of 26 units in which the frequency was increased by the cold stimulation and decreased by warm, and the refractory type of 2 units. The reversed type of the vestibular units was observed only in the unanesthetized cats. In anesthetized animals, about half of the vestibular unitary discharges did not respond to either caloric stimulation, while the refractory type of the units were few in unanesthetized animals. The duration of the increase of the vestibular unitary discharges to caloric stimulation was shorter in anesthetized cats than in unanesthetized ones.