I. A rotation apparatus is described for recording graphically the rotation-nystagmus and the after-nystagmus which occur during and following rotation of rabbits in a horizontal plane. Both rotation-nystagmus and after-nystagmus show a reduction as the result of repeated rotation. The after-nystagmus is more readily reduced than the rotation-nystagmus. Complete obliteration of the after-nystagmus has been observed repeatedly, whereas complete obliteration of the rotation-nystagmus has never been seen. The percentage reduction, both in rotation- and after-nystagmus, is greatest during the first 2 to 4 days of practice rotation. The magnitudes of rotation-nystagmus and of after-nystagmus are dependent upon the duration of the rotation. There is a greater recovery of the after-nystagmus than of the rotation-nystagmus after a rest period. II. In most normal rabbits in which there is a distinct difference in the responses in the 2 directions of rotation, a reciprocal relation is seen between the rotation-nystagmus on turning in one direction and the after-nystagmus in the opposite direction of turning. Thus, if the rotation-nystagmus following turning to the right is greater than that to the left, the after-nystagmus to the left is greater than that to the right. In animals with asymmetrical cerebral lesions, if the after-nystagmus following turning in one direction is affected, the rotation-nystagmus in the opposite direction of turning is also affected. After destruction of one labyrinth, rotation-nystagmus occurs upon turning to the injured as well as to the uninjured side. There is more rotation-nystagmus and less after-nystagmus upon rotation to the uninjured side than upon rotation toward the injured side, and vice versa. It is concluded that in the normal animal upon rotation to the right, stimulation of the receptor structures in the right labyrinth accounts for the major part of the rotation-nystagmus, while the remainder of the effect is due to stimulation of the left labyrinth. On the other hand, the after-nystagmus following rotation to the right is chiefly due to excitations arising from the stimulation of the left labyrinth and, to a lesser extent, of the right labyrinth.