Three-component analysis of caloric nystagmus, focusing on horizontal, vertical, and torsional directions, using a newly developed computerized eye movement analysis system was carried out in ten normal human subjects. The caloric response was induced by cold stimulation to the right ear while the subjects were in supine and prone positions. The directions of the three components of nystagmus, were left in ten subjects (10), upward (8) or downward (2), and clockwise (10) in the supine position. In the prone position, on the other hand, the three components were directed toward the right (10), downward (5), upward (3), and counterclockwise (10) or were not present (2). These findings indicate that caloric stimulation activates the three semicircular canals simultaneously. In addition the changes in the direction of nystagmus in the supine and prone positions could not be explained in detail by the convection theory of caloric response alone.