Recent studies have shown that the interstitial nucleus of Cajal (INC) in the midbrain reticular formation is involved in the conversion of vertical semicircular canal signals into eye position during vertical vestibulo-ocular reflexes. Secondary vestibulo-ocular relay neurons related to the vertical canals, which constitute the majority of output neurons sending signals from the vestibular nuclei directly to the oculomotor nuclei, have been shown to project axon collaterals to the region within and near the INC. To understand how the INC is involved in the signal conversion, latencies of response of neurons in the INC region to electrical stimulation of the vestibular nerve were examined in alert cats. The responses of 96 cells whose activity was clearly modulated by sinusoidal pitch rotation (at 0.31 Hz) were analyzed. These included 41 cells whose activity was closely correlated with vertical eye movement (38 burst-tonic and 3 tonic neurons), and 55 other cells (called pitch cells as previously). Twenty nine of the 96 cells (30%) were activated at disynaptic latencies following single shock stimulation of the contralateral vestibular nerve. Disynaptically activated cells were significantly more frequent for pitch cells than for eye movement-related cells (25/55 = 45% vs 4/41 = 10%; p less than 0.001, Chi-square test). Conversely, cells that did not receive short-latency activation (less than 6 ms) were more frequent among eye movement-related cells than pitch cells (26/41 = 63% vs 13/55 = 24%; p less than 0.001, Chi-square test).(ABSTRACT TRUNCATED AT 250 WORDS)