To assess the effects of long-term changes in aldosterone on potassium distribution within the body, two groups of experiments were conducted. In the first, seven normal dogs received continuous infusion of aldosterone at a high physiological rate, 250 micrograms/day. Total exchangeable potassium (Ke) and plasma potassium concentration (KP) were measured before and 4 and 6 days after aldosterone infusion. KP fell by 20% while Ke decreased by 8% after 6 days of infusion; the ratio between extracellular and total body potassium had been altered by the aldosterone infusion. In the second study, 10 adrenalectomized dogs received aldosterone infusion first at 50 micrograms/day, then at 250 micrograms/day. While on each level of aldosterone infusion, three levels of potassium intake were given by iv infusion. When the animals were in electrolyte balance at each level of aldosterone and potassium (after at least 7 days on each level of infusion), Ke (expressed as meq/kg) and KP were measured. The two variables were plotted against each other, Ke being the independent variable. Data taken while the dogs received 50 micrograms/day aldosterone were described by the equation, KP = 0.100Ke + 0.055, while those obtained at 250 micrograms/day were fitted by the equation, KP = 0.057Ke + 1.30. The correlation coefficients for the two were 0.778 and 0.760, respectively. The regressions were significantly different at a level of P less than 0.02. Data from the two groups of experiments are consistent with the hypothesis that aldosterone alters the distribution of potassium between the intra- and extracellular spaces, a greater portion of total potassium being intracellular at higher levels of aldosterone.