1. The bivalent cation-binding agent, cellulose phosphate, was given for 6 days to four normal subjects and six patients with latent hypoparathyroidism (diagnosed by impaired response to EDTA infusion), all of whom were on a moderately low calcium diet.
2. In normal subjects, there was a prompt and sustained fall in urinary calcium with no change in plasma calcium, indicating increased tubular reabsorption. Plasma and urinary magnesium fell, without increase in tubular reabsorption. The urinary total hydroxyproline increased and Tm,P/ glomerular filtration rate fell after 2 days; these changes were transient and were consistent with a transient increase in parathyroid hormone secretion.
3. In the hypoparathyroid patients, urinary calcium fell more slowly and a fall in plasma calcium occurred in several subjects, the extent and duration of which corresponded with parathyroid status determined by EDTA infusion. Urinary conservation of calcium was impaired but plasma and urinary magnesium fell as in normal subjects. Urinary total hydroxyproline did not change and Tm,P/glomerular filtration rate fell more slowly than in the normal subjects.
4. The relative contributions of increased tubular reabsorption and reduced filtered load to calcium conservation in response to calcium depletion depend on the prevailing level of parathyroid function; the former is more important when parathyroid function is normal, the latter when parathyroid function is impaired.
5. In the detection of reduced parathyroid reserve, the assessment based on the plasma calcium response to cellulose phosphate agrees closely with the assessment based on the degree of recovery from EDTA-induced hypocalcaemia.