The mechanisms of metabolic acidosis and hyperkalemia were investigated in a patient with chronic mineralocorticoid-resistant renal hyperkalemia (5.3–6.9 mmol/l), metabolic acidosis (arterial blood pH 7.27, total CO₂ 17 mmol/l), arterial hypertension, undetectable plasma renin activity ( < 0.10 ng/ml/h), high plasma aldosterone level (32–100 ng/dl), and normal glomerular filtration rate (131 ml/min/1.73 m²). During the hyperkalemic period, urine was highly acidic (pH 4.6–5.0), urinary NH₄ excretion (10–13 μEq/min) and urinary net acid excretion (19–24 μEq/min) were not supernormal as expected from a chronic acid load. During NaHCO₃ infusion, the maximal tubular HCO₃ reabsorption was markedly diminished (19.8 mmol/l glomerular filtrate), and the fractional excretion of HCO₃ (FE HCO₃) when plasma HCO₃ was normalized was 20%. Urine minus blood PCO₂ increased normally during NaHCO₃ infusion (31 mm Hg), and the urinary pH remained maximally low ( < 5.3) when the buffer urinary excretion sharply increased after NH₄C1 load. When serum K was returned toward normal limits, metabolic acidosis disappeared, urinary NH₄ excretion rose normally after short NH₄C1 loading while the urinary pH remained maximally low (4.9–5.2), the maximal tubular HCO₃ reabsorption returned to normal values (24.8 mmol/l glomerular filtrate), and FE HCO₃ at normal plasma HCO₃ was 1 %. Nasal insufflation of l-desamino-8-D-Arginine Vasopressine (dDAVP) resulted in an acute normalization of the renal handling of K and in an increase in net urinary acid excretion. We conclude that: (1) the effect of dDAVP on renal handling of K may be explained by the reversal of the distal chloride shunt and/or an increase in luminal membrane conductance to K; (2) the distal acidification seems to be normal which in the event of distal chloride shunt impairing distal hydrogen secretion might be explained by the presence of systemic acidosis which is a potent stimulus of hydrogen secretion, and (3) metabolic acidosis in the steady state was accounted for by the diminution of bicarbonate reabsorption and ammonia production in the proximal tubule secondary to chronic hyperkalemia.