ARTIFICIAL radioactive isotopes were introduced for turn-over studies on the eye by Kinsey, Grant, Cogan, Livingood and Curtis.1 These investigators used Na24, P32 and Cl38 in their work with rabbits. Recently, Wang2 studied the movement of radioactive sodium and chloride in dogs, whereas Visscher and Carr3 (in dogs) and Scholz, Wilde and Cowie4 (in guinea pigs) limited their study to radiosodium. These investigations revealed the steady state ratios of the tracer elements between plasma and ocular fluids and the turnover rates of these constituents in the aqueous humor. The investigations of Visscher and Carr3 and Wang2 included studies on the rate of entrance of the radioactive elements into the cerebrospinal fluid. All these results confirmed the impression of the usefulness of radioactive elements as tracers in biologic studies. Quantitative data of great accuracy were secured by measuring (with the Geiger-Müller counter)