Since an inverse relationship between waking and REM sleep eye movements (EMs) has been found with respect to frequency, amplitude and direction of EMs, we evaluated variations in the percentages of horizontal rapid eye movements (REMs) during REM sleep after having increased rightward saccades during waking by means of a unilateral tachistoscopic visual test administered prior to sleep. In another condition subjects were given the same 4 h testing condition prior to sleep, the only difference being that they were instructed not to move their gaze; therefore only "attentional movements" were possible. This condition served as a control for the role of oculomotor activity in the complementary relationship between waking and REM sleep eye movements. The actual increase of rightward saccades during waking resulted in a significant decrease of rightward REMs. This effect has been observed only in the condition in which rightward eye movements were increased and it is specific for REMs of amplitude similar to those which were increased during waking by the lateralized test.