The contribution from different pairs of semicircular canals to the generation of horizontal vestibular nystagmus was examined in monkeys. Animals with different pairs of semicircular canals surgically plugged were accelerated sinusoidally at 1 Hz (a predictive stimulus) or with steps of angular velocity (a non-predictive stimulus) about an earth vertical axis while the head was placed in various static pitch positions. In normal animals, and in animals with only the lateral canals intact, horizontal nystagmus elicited with angular velocity steps is maximal at a static pitch angle of 15 degrees nose-down (relative to the horizontal stereotaxic plane). The response follows a cosine function of the pitch angle, approaches zero at an angle of 90 degrees to the optimal orientation, and finally reverses. In animals with only the vertical canals operating, direction specific horizontal nystagmus can still be elicited. Using velocity steps, a null plane at which nystagmus reverses can be determined. It is found at about 32 degrees nose-down and thus is different from the optimal plane of the lateral canals. Consequently, it is not possible to stimulate the lateral canals maximally without stimulating the vertical canals simultaneously. Using sinusoidal rotation, nystagmus is attenuated at the static pitch position of 32 degrees nose-down, but does not reverse direction with further pitching.