Two groups of rats, one hairy and one hairless, received either a magnesium deficient diet (4 mg Mg/100 g diet) or a control diet (40 mg/100 g diet). After four days, an easily observable redness of the skin occurred in the hairless deficient group, progressing from the tail to the forehead, and later a hyperemia of the ear and dermatosis which increased with scratching appeared in both deficient groups. The clinical signs were more acute in the deficient hairless group. Histamine, total white blood cells and eosinophil counts increased in both deficient groups during the allergy-like crisis. Then, these factors tended to normal values. After three weeks on a diet an enlargement of the spleens was observed, but not of the thymus in both deficient groups. There was a large difference in the histamine content of the spleen between the deficient and control hairless groups: the values obtained were 4 350 ng and 166 ng per 100 g tissue respectively.