The cutaneous vasodilatation occuring in the early stages of dietary deficiency of magnesium has been investigated in rats. While the time of onset of erythema varies in proportion to the weight of the animal, the duration is not related to weight. In severe states of vasodilatation, the skin is thickened and infiltrated with mononuclear cells, apparently derived from the blood. Intact sympathetic and sensory innervation are not necessary for the development of vasodilatation in the skin. Neither can the genesis of the erythema be attributed to degranulation of mast cells. From consideration of this and other investigations, it is concluded that the cutaneous abnormalities of magnesium-deficient rats cannot be due directly to hypomagnesaemia.