Multiunit activity (MUA) in the suprachiasmatic nucleus (SCN) and adjacent hypothalamic areas were compared in albino rats anesthetized with thiopental, immobilized with succinylcholine chloride and artificially respired. During adaptation to light lasting up to 2 h, MUA in the SCN increased gradually, whereas MUA in the other locations outside the SCN decreased. During adaptation to the dark, MUA in the SCN decreased whereas MUA outside the SCN increased. This reversal was apparent not only when recording in the SCN and adjacent hypothalamus simultaneously, but also while advancing an electrode from the optic chiasm through the SCN to the anterior hypothalamus.