Three-dimensional analysis of vestibulo-ocular reflexes, using a computerized eye movement analysis system, was carried out in 16 normal human subjects. We examined the relationship between head position and vestibulo-ocular reflexes. The purpose of this study was to clarify the relationship between the functional plane and the anatomical position of the semicircular canals. The subjects were stimulated by pendular rotation with their heads fixed between 40 degrees nose down and 50 degrees nose up in 10 degrees steps with respect to the orbitomeatal line and the lateral canal line. The gain in horizontal eye movement which results from the lateral canal, was the highest at 20 degrees nose down. On the other hand, the gains in vertical and torsional eye movement, which result from the vertical canals, were the lowest at 10 degrees nose up. The phase of vertical and torsional eye movement changed 180 degrees at 10 degrees nose up. These data indicate that the functional plane of the lateral canal almost coincided with the anatomical position. However, at this functional plane of the lateral canal, the influence of vertical canals was not the least. We suggest that this could be the reason why angles formed by the intersection of the lateral canal plane and the vertical canals were not 90 degrees. We could interpret this finding as showing that at the 10 degrees nose up position, the origin of eye movement alternated between the frontal and posterior canals.