1. A group of rabbits was actively immunized against aldosterone and their subsequent response to infused aldosterone and to dietary sodium restriction was compared with that of a control group.
2. Colonic electrical potential difference (CP) was used as an index of relative mineralocorticoid activity in both groups of animals.
3. Infusion of aldosterone produced a rise in CP to a maximum of −50 to −70 mV at rates of infusion of 55–270 pmol min−1 kg−1. In contrast, the immune group showed no change until infusion dose rates of 2700 pmol min−1 kg−1 were used.
4. During dietary sodium restriction, CP increased in normal rabbits to a significantly greater degree than in immune animals although cumulative urinary sodium losses were the same in both groups.
5. The ability of the immune animals to remain in sodium balance despite physiological aldosterone blockade suggests that aldosterone is not an essential component of their sodium-conservation mechanisms.