Seventeen patients with spontaneous or postsurgical parathyroid insufficiency presented several electroencephalographic manifestations, which were present individually or in combination: a marked tendency of the a-rhythm to diminish or disappear in the occipital, parietal, and to a lesser degree, in the frontal records; an increase in constancy and amplitude of the B-rhythm; it can be uniform, paroxysmal, or appear as a modulation of the a-rhythm; presence of simple spikes; groups of 6-7 slow waves (2-3 per sec.) appearing mostly in the fronto-parietal leads. In cases in which medical treatment had corrected the signs of clinical insufficiency, a hyperventilation of 45 secs. evoked in the EEG the above-mentioned signs. Parathyroid grafts (glands of human newly-born) corrected the clinical and electro-encephlalographic manifestations of insufficiency. In the cases in which the graft was re-absorbed the signs gradually reappeared. When the graft remained active (3 yrs. in one case) the EEG was normal even after hyperventilation.