AbstractElectrocortial synchronization, associated behaviourally with light sleep, was induced by low‐frequency stimulation of the subcallosal region and the orbital gyrus of the frontal lobe in unanesthetized cats. The effect was weaker than that obtained with basal forebrain stimulation. The above cortical areas have previously been shown to exert strong inhibitory influences on a variety of somatomotor and autonomic activities. High‐frequency stimulation of the same areas produces electrocortical desynchronization and behavioural arousal. It is suggested that the subcallosal and orbital portions of the frontal cortex give origin to.a descending system capable of modulating the excitatory state of sleep‐producing mechanisms in the brain stem or diencep