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A comparison was made of the distribution and excretion of the optical isomers of thyroxine in the rat. d-Thyroxine disappeared more rapidly from the blood and was concentrated to a greater extent in the liver and kidney than was l-thyroxine. The concentration of d-thyroxine in such peripheral tissues as muscle, brain and skin was much less than the concentration of l-thyroxine. This difference in distribution to the peripheral tissues was of such magnitude as to suggest a possible basis for the different activities of the two isomers observed in vivo. The half-time of d-thyroxine in the body (14 hr.) was significantly shorter than that of l-thyroxine (42 hr.).

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