Adrenergic alpha 2 agonist (clonidine) and its antagonist (yohimbine) were locally applied to the medial preoptic area (mPOA), to find out the role of alpha 2 receptors at this brain region in the regulation of sleep-wakefulness. Clonidine produced arousal, whereas yohimbine induced sleep in freely moving animals. Behavioural arousal produced by clonidine administration was accompanied by EEG synchronization. The alpha 2 receptor as the probable site of action of externally applied norepinephrine (NE), is discussed.