Plethysmography may be used as a practical tool for objectifying the syndrome of slight encephalopathies in children. Our results show that small anatomical lesions associated with the MBD syndrome are localized subcortically and particularly in reticular formations, as had already been shown by O. Kucera in 1961. It appears that some specific disorders associated with the MBD syndrome are dependent upon the localization of the intensity of the lesions in basal ganglia rather than upon an etiological specificity. The observation of the vasomotoricity may give an insight into complicated physiopathological processes of the MBD syndrome, without however identifying the disorder with the vasomotoricity. We are therefore of opinion that a further development of this kind of investigation into the MBD syndrome may be helpful to a reliable and greater accuracy of the objective diagnosis.