We examined the independent effects of a high potassium diet and increased aldosterone levels on the development of renal potassium adaptation. This condition is defined by the increased ability of the kidneys to excrete an acute infusion of potassium. Rats were adrenalectomized (ADX) and received aldosterone at basal levels (0.5 Asg/100 g * d) or at high levels (2.0 ;g/100 go d) for 10 d. In each experimental group, animals received either a control diet or a high potassium diet. In ADX animals with basal aldosterone levels, a high potassium intake increased but did not completely restore the ability to excrete potassium and induced proliferation of the basolateral membrane of principal cells in the collecting tubule (i.e, morphologic adaptation). In contrast, increased aldosterone did not induce functional adaptation. Elevated aldosterone and dietary potassium intake were required to produce functional potassium adaptation indistinguishable from that in potassium-loaded, adrenal-intact animals.