The hypertensive effect of the acetates of d-aldosterone and deoxycorticosterone was compared by injecting 0.125 mg of each twice daily in oil subcutaneously into unilaterally nephrectomized rats given 1% saline solution to drink. The two hormones had entirely comparable activity in respect to the enhancement of saline consumption, the development of hypertension and the magnitude of cardiac enlargement induced. Aldosterone treatment caused a much greater degree of renal hypertrophy and far more severe vascular lesions in the heart and kidney. It is suggested that both of these effects may reflect the superior ability of aldosterone to cause potassium excretion and therefore a more severe hypokaliaemia. Only aldosterone caused thymic involution, believed to be an indirect response, and impairment of body growth. Under the circumstances of this experiment the hypertensive potency of aldosterone was at least as great as that of deoxycorticosterone, and the ability to bring about vascular damage far greater. Although it is recognized that this relationship might not obtain at all dose levels of the two steroids, it is suggested that the lesser activity usually ascribed to aldosterone as compared with DCA when the two are given in dosages calculated to cause an equivalent degree of sodium retention, may reflect the operation of variables such as absorption rate, enzymatic inactivation rate and hence the respective quantities in the circulation at any given time, rather than differences in the inherent potency.