The object of this investigation was to determine the rate at which water enters and leaves the eye and, more particularly, to discover the rate of total water movement between the aqueous and its environment. To accomplish this, it is essential to distinguish one molecule of water from another, for water molecules in the eye are continuously exchanging with those in the blood stream, irrespective of any net flow. Because so-called heavy water (D2O) cannot be distinguished physiologically from ordinary water, it is ideally suited for following the course of water molecules in biologic materials. In the absence of any means of labeling the water molecules, all previous investigators of ocular fluid dynamics have been limited to measuring the net flow of liquid. It will be shown subsequently that the net flow represents but a small fraction of the total water movement within the eye; the total movement