According to several morphological and functional findings, strict similarities seem to exist between the magnocellular portion of the medial geniculate body (GMmc) and the brain stem aspecific structures. In view of this, the possibility has been investigated, that GMmc might exert some measure of control on the diffuse electrocortical activity. Both acute and chronic experiments were performed. The acute experiments were carried out on locally anaesthetized, curarized cats using stereotaxic techniques and EEG recording. Both in neuraxially intact and in encéphale isolé preparations, repetitive stimulation (concentric electrodes, 0.5-0.7 mm in outer diameter) of GMmc was seen to induce electrocortical desynchronization if high frequencies were used (200-300/sec), whereas diffuse synchronizing effects became apparent using low frequencies (4-12/sec). In the same preparations localized cooling of GMmc (CO₂ - thermode, 1 mm in outer diameter; 10° C for 0.5-5 min) elicited diffuse and reversible EEG activation; moreover, the activating effect could be reproduced by local microinjection of 3M KCl (1-3 $\mu l$). Chronic experiments were carried out on freely-moving cats with implanted EEG, EMG and EOG electrodes as well as with multiple stimulating electrodes for GMmc and adjoining structures. The results obtained were fully confirmatory of those observed in acute preparations. Taken together, the present findings would suggest that GMmc does exert some diffuse control on the level of electrocortical activity, with a prevalence of synchronizing action in view of the results of inactivation experiments. Control experiments support this conclusion.