Groups of rats were given either a control or a magnesium-deficient diet. The well-known allergy-like crisis, characterized by vasodilatation with redness of the ears and dermatosis, occurred spontaneously in the magnesium-deficient groups. The histamine (H) content and the distribution of the various white blood cells (WBC) were studied as a function of time. During the acute phase, there was a transitory elevation of total blood H and WBC, mainly affecting the polymorphonuclear (PMN) cells, eosinophils (EO) and basophils (BAS). The EO peak preceded that of H. The EO count was especially high during the first part of the acute phase, while H and BAS were higher during the second part of that phase. There were no BAS's in the blood of the controls and those in the Mg-deficient animals were only partly granulated. H, EO and mast cells (MC) were elevated in the spleen but not in the thymus during acute deficiency. The high H level in the spleen corresponded to the same high level in the blood. When the spleen suspensions were centrifuged, wide differences in supernatant and pellet histamine appeared, according to the deficiency period.