EEG rhythms recorded during the various states of alertness and sleep--waking cycles were studied in 4 baboons before and after optic nerve section. Visual deafferentation induced a general increase in amplitude of all the cortical rhythms, with general accentuation of pre-existing activities such as occipital eye movement potentials (EMPs), the fronto-rolandic rhythm and the alpha rhythm, spindles, REM ponto-geniculo-cortical (PGC) spikes; an increase in amplitude of the geniculate rhythmic activity, its frequency and occurrence was also noticed. When the blind baboons were placed in a monotonous environment, sleep--waking cycles occurred at any time of the 24 h. Total sleep time and total REM duration were not changed; however, deep sleep (stages 3 and 4) duration decreased (by around 50%) whereas stages 1 and 2 increased. The light factor thus plays a role in the sleep--waking cycle organization. However, its influence does to seem to be essential since its loss can be compensated by auditory stimulation. Finally, light possibly influences the systems involved in slow wave sleep regulation.