The interaction of smooth pursuit eye movements, vestibulo-ocular reflex (VOR) and optokinetic reflex (OKR) is still not well understood. We therefore measured in macaque monkeys horizontal eye movements using transient horizontal rotations of a visual target, of monkeys' heads and/or of an optokinetic background pattern (ten combinations; smoothed position ramps of 16 degrees ). With intermediate peak velocity of target motion (v(max)=12.8 degrees /s), pursuit held the eyes rather well on target, almost independent of concurrent vestibular or optokinetic stimuli (pursuit gain, 0.73-0.91). With v(max)=1.6 degrees /s, in contrast, pursuit gain became strongly modified by the optokinetic stimulus. With v(max)=51.2 degrees /s, pursuit gain became modified by vestibular stimulation. Although not intuitive, the experimental data can be explained by linear interaction (summation) of the neural driving signals for pursuit, VOR and OKR, as ascertained by simulations of a dynamic model.