Vestibular brain stem units of the horizontal canal in the awake C1 transected cat were examined in acute experiments before and after the contralateral vestibular nerve was cut. The latter procedure resulted in an immediate increase in mean resting rate from 19 to 45 spikes/sec and greater spontaneous fluctuations in firing rate. Mean sensitivity to constant accelerations from 2 to 16 degrees/sec2 was significantly reduced. In the 4-6 degrees/sec2 range, for example, mean sensitivity fell from 5.7 +/- 3.2 spike/sec/deg/sec2 (range 0.7-13.6, n = 107) to 2.8 +/- 1.4 (range 0.2-5.5, n = 29). Mean sensitivity to 4-6 degrees/sec2 deceleration was reduced from about 4.5 to 3.0. On the other hand, time constants and the ratio of adapting to non-adapting units was unchanged. Removal of the midline cerebellum, including the vermis and fastigial nuclei in a portion of the animals, did not materially alter the above results. It is concluded that the contralateral labyrinth, acting via the vestibular brain stem commissural pathway, not only exerts a powerful effect on the resting activity of brain stem canal vestibular neurons, but also on their response to acceleration.