Currently, the vestibulo-ocular and optokinetic reflexes are both thought to require a velocity storage mechanism within their neural pathways. To test whether these storage mechanisms are shared by both reflexes, animals were given programs of stimulation known to change the status of the storage mechanism of one of the type of nystagmus. The other type of nystagmus was examined then to ascertain whether the characteristic of its velocity storage system had remained invariant. Horizontal eye movements of three macaque monkeys were recorded during post-rotatory nystagmus and optokinetic afternystagmus (OKAN) before and after 20 habituating exposures to either vestibular or optokinetic stimulation. Repetitive exposures to vestibular stimulation alone markedly reduced the time constant of post-rotatory nystagmus and this effect was accompanied only occasionally by a reduction in the time constant of OKAN. Repetitive exposure to optokinetic stimulation alone reliably reduced the time constant of OKAN but produced no reliable change in the time constant of post-rotatory nystagmus. These results suggest that the vestibulo-ocular and optokinetic reflexes do not share a single common velocity storage mechanism.