This study was designed to investigate the discharge patterns of firing neurons on the primary afferent vestibular nerves and their caloric responses. 15 guinea pigs were used for the experiment and the head of the animal was fixed in a stereotaxic instrument which could be rotated 3-dimensionally and tilted anteroposteriorly and laterally. A small portion of the cerebellum and occipital lobe was aspirated to expose internal auditory meatus. A glass-microelectrode filled with 3M-KCI was inserted into the Scarpa's ganglion. Vestibular nerve afferents were classified as regular or irregular discharging in terms of coefficient of variation(CV). The end organ which an afferent innervated was identified. The changes in the firing rates of the unit were observed on the bithermal stimulation. The proportion of irregular neurons was much larger than that of regular ones and the caloric response was more prominent in irregular type of neurons. On horizontal face down positions, lateral semicircular canal responded most prominently to caloric stimulation, followed by anterior semicircular canal and otolith in order. From the above results, irregular type of neurons seemed to be more sensitive than regular ones.