Evidence is accumulating that suggests that the "pump-leak" model which has served as a useful framework for interpreting cation changes in the lens needs to be modified. A new model is proposed. The lens lumen of the embryo becomes the fibroepithelial space after the posterior cells have elongated into fibers. The space is lined by the apical membranes of the epithelial cells and fibers. The lens is viewed as having a structure that is designed to maintain a salt and water balance with a minum expenditure of energy. Hence, the basolateral membranes are relatively impermeable while the apical membranes are permeable and work with the Na,K-pump, that is located in the lateral membranes, to remove (sodium) or conserve (potassium) substances that enter the lens through the intercellular spaces. Potassium enters the fibroepithelial space by way of the pump and the apical membranes of the epithelial cells. As it diffuses outward through the intercellulae spaces, it is recycled until the Na,K-pump is saturated. Hence, the concentration of potassium in the fibroepithelial space is elevated. Potassium is distributed to the bulk of the lens as a "passive" Donnan equilibrium across the apical membranes. The sodium concentration is maintained at a low level so that it acts like an "impermeable" ion and, hence, plays a major role in maintaining lens volume. The concentration of potassium in the lens is of importance for protein synthesis and enzyme activity. The model that explains the basis for maintaining the lens potassium concentration, therefore, is of significance for interpreting changes noted in experiments or pathological states and for designing new investigations or mathematical paradigms. An attempt will be made to substantiate the proposed model by demonstration an equilibrium distribution of potassium in normal rat lenses and by studying cation concentrations and responses to perturbation in lens with patent lumens - embryonic chick lenses and regenerating rat lenses following the production of mature galactose cataracts.