In eight patients with advanced renal insufficiency (inulin clearance 1.4–9.1 ml/min), concentrations of serum calcium (S[Ca]) and phosphorus (S[P]) were maintained normal (S[Ca] > 9.0 mg/100 ml, (S[P] < 3.5 mg/100 ml) for at least 20 consecutive days with phosphate binding antacids and oral calcium carbonate. The initial serum levels of immunoreactive parathyroid hormone (S-PTH) were elevated in three (426–9230 pg/ml), normal in four (one after subtotal parathyroidectomy), and not available in one. The initial fractional excretion of filtered phosphorus was high in all and ranged from 0.45–1.05. Following sustained normo-calcemia and normo-phosphatemia, S-PTH was reduced below control levels in all patients; being normal in six and elevated in two. decreased below control levels in all patients; it remained high in six (of which five had normal S-PTH) and was normal in two (of which one had elevated S-PTH). The observed relationship between S-PTH and could either reflect the inability of the radioimmunoassay for PTH employed to measure a circulating molecular species of PTH which was present in which case the actual levels of S-PTH were higher than those measured, and/or it could be indicative of the presence of additional important factor(s) (other than S-PTH) which inhibit tubular reabsorption of phosphorus in advanced chronic renal failure.