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Patients with transient ischemic attack (TIA) in the vertebrobasilar artery more often complain of disturbance of equilibrium, such as vertigo or dizziness, than of auditory disorders, such as hearing impairment or tinnitus. The author induced TIA in rabbits by injecting adenosine diphosphate (ADP) into the right vertebral artery. Observations of rotatory nystagmus have shown that a peak level of directional preponderance appeared 1 minute after injection of ADP (0.3mg/kg b.w.) and continued for more than 4 minutes. On the other hand, the amplitude of auditory brainstem responses (ABRs) presented no significant changes following the injection of ADP (0.5mg/kg b.w.). However, greater amounts of ADP (1.0 and 2.0 mg/kg b.w.) were found to reduce the amplitude of ABR-waves, although significant reduction was observed for only less than 1 minute. This change was represented by flattening of the later part of the waves, with the first wave much less affected. Electrocochleogram (E. Coch. G.) have also demonstrated a transient reduction in amplitude 10 seconds after injection of ADP (4.0mg/kg b.w.) with correspondingly rapid recovery. In cases of hypertension due to noradrenaline load or trimethaphan-induced hypotension, reduced amplitudes were more profound and continued longer, returning to its normal range within 1 minute. Differences in susceptibility between the equilibrium system and auditory system in TIA of the vertebrobasilar artery are discussed.