We have investigated the contribution of ocular muscle proprioception (OMP) to the long-term maintenance of ocular alignment in normal human beings. Using a scleral suction lens, one eye was rotated laterally 30 deg away from the position of the other eye. This procedure selectively affects OMP without altering the efferent copy of the ocular motor command. The passively displaced eye was covered while the unimpeded eye fixed upon a stationary target. The suction lens was removed after 6 or 10 min and the measures of alignment begun immediately. Three tests were used to determine the effects of the deviation on ocular alignment: the Lancaster red-green test; saccadic eye movement responses to stepping targets; and hand pointing to monocularly presented targets. All three tests indicated a change of ocular alignment of about 2-4 deg, lasting 5-10 min: sustained temporal deviation resulted in exophoria (relative divergence of the visual axis), and sustained nasal deviation induced esophoria (relative convergence). Binocular viewing rapidly abolished the effect. The hand pointing test showed a large shift in the perceived position of a target during monocular viewing with either eye and its amplitude was correlated with the change of ocular alignment. These results indicate that a sustained passive rotation of one eye can lead to a persistent change in ocular alignment even after the eye is released, without any disparity cues. We further suggest that central mechanisms, based upon ocular motor afferents, rather than passive orbital mechanical factors, are the main cause of this phenomenon.