THE behaviour of the kidney in relation to the electrolytes of the extracellular fluid is usually described in terms of two mechanisms operating to maintain the homceostatic state : (a) Regulation of the concentration of several individual electrolytes and non-electrolytes within a limited range of variability. The concept of ‘renal threshold' is implied in this proposition, (b) Regulation of the osmotic pressure of the extracellular fluid, so as to maintain isotonicity with intracellular fluid.