In order to know the effects of caloric stimulation on neuronal firing in medial vestibular nuclei (MVN) by middle ear irrigation, the middle ear was irrigated with ice (4 degrees C), hot (44 degrees C), and warm (37 degrees C) water, and the firing rate of MVN neuron was extracellularly recorded. The results showed that the firing rate of MVN neuron was changed by caloric stimulation, and the majority of MVN neurons showed excitation by irrigation with hot water and inhibition by ice water (type A). The neuronal firing was recovered immediately after the cessation of the stimulation. I It was concluded that the neuronal firing rate in MVN was changed by caloric stimulation in middle ear cavity. The response was different in various neurons.