In vivo chronic studies were made of the microcirculation of the mouse ear. Using a mapping technique it is possible to make repeated observations of single elements, circulatory, pilosebaceous, etc. Vasodilatatlon can be ascertained by measuring the diameters of a constant sample of blood vessels. The presence of protelnaceous edema may be detected by the visual obliteration of the cartilage cells by the edema fluid. Edema was produced by plucking the hairs or by applying a chemical depilator for varied time periods. Vasodilatation usually accompanied the edema but not invariably. The dilatation was more characteristic of arteries, veins and arterloles than of capillaries or small venules. Vasodilatation was also produced by heating, and in this case it was never followed by edema. Summation of possible cases showed that 45% evinced a separation of occurrence of vasodilatation and edema. This indicates that the 2 phenomena may exist independently, and are not inevitably associated together as has been maintained by some workers in the field.