The purpose of this study was to examine renal potassium handling in patients with low aldosterone bio-activity. The patients with a normal renal response to aldosterone were identified by finding both a low plasma aldosterone concentration during hyperkalaemia and a transtubular potassium concentration gradient (TTKG) in the cortical distal nephron of 6 or greater within 4 h after the administration of a physiologic dose of mineralocorticoid hormone. In contrast, patients with a primary renal potassium excretion defect represent a heterogenous population. In some, the TTKG rose when a pharmacologic but not a physiologic dose of mineralocorticoid was given; others had little renal response to the administration of this hormone. Furthermore, this renal response may be delayed and require more than 24 h to become manifest.