This study was performed on 18 albino rats subjected to either unilateral (11) or bilateral (7) electrolytic destruction of the midbrain reticular formation. In all preparations a complete unilateral or bilateral interruption of the ascending activating system was achieved. The electrocorticographic pattern of synchronized sleep ocurred in both unilaterally and bilaterally lesioned rats. This was found as soon as the animal recovered from the surgical trauma, when slow waves still predominated. Such finding support the hypothesis that activating systems other than the midbrain reticular activating system exist whose functional block during sleep furthers the synchronization of the cortical electrical activity. An increasing recovery of desynchronization and motor activity as a function of time was shown by all rats. However, recovery occurred earlier and more quickly in the animal with unilateral lesions. Physiological sleep evolved with all its phases by the fourth or fifth day after the lesion was made. It seems that desynchronization of the paradoxical phase of sleed does not involve any particular struture arising from and/or passing through the midbrain tegmentum.