The pink color of specimens exposed to a red background is due to a vasodilatation. This phenomenon is seen at its maximum by the 3rd-5th day of exposure to the stimulus if the specimens are contained in fresh water. When salt water is used the pink color is not evident, the vasodilatation being inhibited though not completely; for there is here also a slight dilatation especially if the stimulus is prolonged, but it is not sufficient to give the pink color so apparent in specimens placed in fresh water. When the specimens are changed to a neutral or white background the vasodilatation largely disappears within a few minutes. A close approximation to the condition produced by the red environment in Fundulus was obtained by the use of adrenalin. An intra-peritoneal injection of 0.50-0.75 cc. of adrenalin chloride (dilution 1:1000) caused a vasodilatation within 5-6 minutes, and at a later phase the coloration was similar to that obtained when under the influence of a red environment.