Recent studies indicate that the direction of postrotatory nystagmus and optokinetic afternystagmus reorients toward earth-horizontal in tilted subjects. To further examine this phenomenon in humans, we studied 8 adults (4M, 4F) with no history of neurologic or otologic disease. Vestibular stimulation consisted of yaw off-vertical axis rotation (OVAR) trapezoids with eyes open in the dark at 60 degrees/s constant velocity with a tilt angle of 30 degrees and a deceleration at 100 degrees/s2 to a stop in either the right-ear-down or the left-ear-down position. The optokinetic stimulus consisted of 5 degrees wide black and white stripes projected against a cylindrical visual surround 1 m in diameter rotated at a constant velocity of 30 degrees/s. The projector, visual surround, and subject were all tilted by 30 degrees; subjects were placed either in the right-ear-down or left-ear-down position. Eye position was measured using the magnetic scleral search coil technique. Each trial was scrutinized for the presence of reorientation of nystagmus in the subject's roll plane by looking for vertical nystagmus in the appropriate direction. Results indicated that reorientation of postrotatory nystagmus following OVAR was variable but often present. Reorientation of optokinetic afternystagmus was neither as consistent nor as robust as that seen following OVAR. These findings confirm that humans exhibit a predilection for eye rotations in the earth-horizontal plane.(ABSTRACT TRUNCATED AT 250 WORDS)