To establish the difference of mechanism between irritative and paralytic nystagmus, alterations of Na-K-ATPase and succinic dehydrogenase activity in the vestibular sensorineural elements were investigated for 20 guinea pigs, and glucose uptake of the vestibular nuclei for 13 guinea pigs were measured by the [14C]-2-deoxy-D-glucose method. Irritative and paralytic nystagmus were experimentally provoked by introducing K+ into the perilymphatic space. From the results it was concluded that irritative nystagmus is provoked by increased excitability of vestibular sensory cells, while paralytic nystagmus is provoked by decreased excitability. However, the direction of nystagmus was eventually decided by the tonus imbalance between the bilateral vestibular nuclei. The ipsilateral vestibular nucleus was predominant during irritative nystagmus, while the contralateral vestibular nucleus was predominant during paralytic nystagmus.