Bilateral transections across the brainstem interrupted the medial longitudinal fasciculus (MLF) in three monkeys trained to make eye movements while subjected to horizontal or vertical angular accelerations. Eye movements measured before and after the lesion revealed deficits in both voluntary and vestibular compensatory eye movements; the deficits differed in the horizontal and vertical directions. Vertical saccades in both directions were normal but eccentric positions of fixation could not be maintained; a drift toward the midline followed by a corrective saccade produced vertical fixation nystagmus. Furthermore, the vertical vestibulo-ocular reflex (VOR) was abolished and vertical smooth pursuit was impaired. Along the horizontal meridian, adduction across the midline could not be achieved during either saccades, smooth pursuit, or the VOR. Temporal saccades were normal whereas nasal saccades were considerably slowed. If the eye was not required to cross the midline, the phase shift during the VOR was within 15 deg of normal in each eye. The gain of the VOR was reduced to about 0.4 immediately after the lesion, but recovered within one month. These findings suggest that the MLF transmits quite different kinds of information to horizontal and vertical oculomotoneurons and that deficits in vertical eye movements may be a sensitive indicator of anterior internuclear ophthalmoplegia.