The vestibular system responds to head acceleration by producing compensatory reflexes in the eyes and postural muscles. In this article we investigated the effect of brief interaural acceleration on the vestibular evoked myogenic potential (VEMP) in 10 normal subjects and 10 patients with bilateral (bVL) or unilateral (uVL) vestibular loss. Stimuli were delivered with a handheld motor and tendon hammer over the mastoid and produced relatively pure interaural head acceleration with little rotation (mean peak acceleration 0.14 g at 3.3 ms). VEMPs were recorded from the neck muscles, and were characterised in normal subjects by a positive/negative potential ipsilateral to the stimulated side (peak latencies 15.1 and 22.6 ms) and a positive response contralaterally (20.3 ms), which was sometimes preceded by a negativity (14.5 ms). These peaks were absent in patients with bVL, confirming their vestibular-dependence. In the patients with uVL, medial acceleration of the intact ear produced bilateral responses, an initial positivity on the intact side and a negativity on the affected side, while lateral acceleration produced only a late positivity on the intact side. As the acceleration was primarily in the horizontal plane it is likely to have activated utricular receptors. Consistent with this, we found that VEMPs are very sensitive to the direction of head acceleration and have features consistent with the utriculo-collic projections demonstrated in animals.