An increase in Na-K-ATPase in kidney homogenates usually accompanies compensatory renal hypertrophy. While it may be evident in both the cortex and medulla of the kidney, it is most marked in the outer medulla and may be present only in that region. The increase in enzyme activity does not depend on an intact adrenal cortex and can be elicited in the absence of adrenal glucocorticoids. It is not seen in the form of renal hypertrophy produced by potassium depletion, in which the transport of sodium and potassium by the kidney is not increased. When present in compensatory renal growth, the enzyme change is correlated with an increase in the reabsorption of sodium, or the excretion of potassium, or both, per unit of renal tissue. It proceeds in the presence of either, but not in the absence of both.