Bilateral lesions of the midbrain reticular formation within, and in the close vicinity of, the interstitial nucleus of Cajal (INC) result in the severe impairment of the ability to hold eccentric vertical eye position after saccades, phase advance and decreased gain of the vestibulo-ocular reflex (VOR) induced by sinusoidal vertical rotation. In addition, the INC region of alert animals contains many burst-tonic and tonic neurons whose activity is closely correlated with vertical eye movement, not only during spontaneous saccades, but also during the VOR, smooth pursuit and optokinetic eye movements. Although their activity is closely related to these conjugate vertical eye movements, it is different from the oculomotor motor neuron activity. These results indicate that the INC region is involved in, and indispensable for, some aspects of eye position generation during vertical eye movement. Further comparison of INC neuron discharge with eye movements during two special conditions indicates that the INC region alone cannot produce eye position signals. First INC neuron discharge shows no response or an 80 degrees phase advance (close to the expected value if there is no integration) in the dark compared to the light during sinusoidal vertical linear acceleration in alert cats. Second, during rapid-eye-movement (REM) sleep, the discharge of INC neurons is no longer correlated with eye position. These results imply that the INC is not the entire velocity-to-position integrator, but that it has to work with other region(s) to perform the integration. A close functional linkage has been described between vertical-eye-movement-related neurons in the INC region and vestibulo-ocular relay neurons related to the vertical semicircular canals in the vestibular nuclei. It has been suggested that both are the major constituents of the common neural integrator circuits for vertical eye movements.