A twelve-year-old girl with persistent hyperkalemia, metabolic acidosis, normal blood pressure and glomerular filtration rate, and short stature (first percentile for height) was studied using metabolic balance techniques. Prior to therapy with hydrochlorothiazide (HCTZ), urinary potassium and acid excretion were low and urine pH was inappropriately high at 5.8. HCTZ (25 mg orally per day) (1 mg/kg) was then started and rapidly corrected her serum electrolytes. The therapy with HCTZ was associated with a diuresis, a decrease in urine pH to 4.8, and concomitant increases in potassium, titratable acid (TA) and ammonium excretion. The increase in TA excretion was explicable, in part, to the decrease in urine pH and, in part, to the considerable increase in phosphate excretion (from 56 to 81 mmol/d). Plasma renin activity and plasma aldosterone increased markedly following HCTZ but urinary prostaglandin E (PGE) excretion was unchanged. These observations suggest that administration of HCTZ in this setting increases hydrogen ion secretion. It is unclear whether this effect is a direct consequence of HCTZ at the level of the tubule or is secondary to some other action of HCTZ. However, it is clear that this effect is not related to an alteration in PGE excretion.