Low-frequency repetitive stimulation of the fastigial nucleus (concentric electrodes; 5-15/sec; 0,1-3 msec; 0,7-2,5 V) was seen to exert in the cat a definite synchronizing action on spontaneous electro-cortical activity, both in neuraxially-intact, unanesthetized animals (local 2% procaine), and in encéphale isolé preparations. The effect is opposite to the well-known arousal response elicited by high-frequency stimulation (100-300/sec) of the same nucleus. It occurs always bilaterally and it generally involves the whole extent of the hemispheral cortex, although in some cases the response predominates in the occipital regions. From the electrographic standpoint, the effect is sharply different from the recruiting response since it is marked by an increase of spontaneous spindling or by the appearance of stimulus-unrelated, high-voltage slow waves. As was already described for other synchronizing structures, the occurrence of such effect depends to some extent on the functional level obtaining in the cortex before the stimulus. The response appears at the beginning of the stimulation or after some delay, and often outlasts the stimuli for seconds or even minutes; it can still be evoked after bilateral interruption of the superior cerebellar peduncles. The functional significance of the results reported is briefly discussed.