The classical studies of Lewis1 and Dale2 clearly established the role of histamine in the reactions of the skin to injury. It is all the more surprising, therefore, that until recently virtually nothing has been known of the cytological location of tissue histamine. According to Riley,* this gap between the physiology and the pharmacology of histamine, on the one hand, and the morphological distribution of histamine, on the other, can be bridged, if it be assumed that much of the histamine in the body is normally present in the mast cells, a hypothesis for which there is now considerable experimental support.† It was thus of interest to examine the skin of various species and to determine the extent to which the distribution of the histamine in each is related to the distribution of its mast cells. This study shows that at least