The interacting effects of aldosterone and plasma potassium concentration on steady-state renal potassium excretion were studied in two groups of chronically adrenalectomized dogs. In group I (six dogs, 22.9 kg) aldosterone was infused intravenously at 20 micrograms/day while potassium intake was changed in steps of 7-10 days duration from 10 to 30 to 100 meq/day. At the completion of each step, plasma potassium concentration, urinary potassium excretion, and other variables that potentially may affect renal function were measured. In group II (six dogs, 22.2 kg) a similar protocol was followed except that aldosterone was infused at 250 micrograms/day and the potassium intake levels were 30, 100, and 200 meq/day. Plasma potassium concentration and excretion data for the 20 micrograms/day group were: 3.22 +/- 0.26 meq/liter and 5 +/- 1 meq/day, 4.35 +/- 0.08 meq/liter and 21 +/- 2 meq/day, and 5.88 meq/liter and 82 +/- 3 meq/day at the 10, 30, and 100 meq/day intake levels, respectively. For the 250 micrograms/day group the values were: 2.72 +/- 0.18 meq/liter and 28 +/- 7 meq/day, 4.16 +/- 0.14 meq/liter and 71 +/- 8 meq/day, and 4.40 +/- 0.14 meq/liter and 172 +/- 26 meq/day at the 30, 100, and 200 meq/day intake levels. Therefore, the increase in aldosterone infusion rate shifted the relationship between plasma potassium concentration and potassium excretion to the left so that at a given level of plasma potassium a greater amount of potassium was excreted. In the normal range of plasma potassium concentration (4.00-4.40 meq/liter) the increase in aldosterone levels resulted in a four- to eightfold increase in daily potassium excretion.