Effects of oral administration of phosphorus on phosphorus and calcium parameters and on bone metabolism hormones were investigated in two studies: in the first, 12 young female adults and 8 females over 50 years of age took a single oral load dose of 780 mg elemental phosphorus, whereas in the second study assays were performed 5 and 15 days after initiation of twice-daily phosphorus supplements in 19 young adults, 14 subjects above 50 years of age who were free of osteoporosis, and 12 patients with osteoporosis also over 50 years of age. The phosphorus load was associated with an increase in intact parathyroid hormone levels (iPTH 1-84), which was more marked in subjects above 50 years of age than in younger adults. On the 15th day of phosphorus supplementation, there was no significant increase in baseline iPTH and no change in baseline dihydroxyvitamin D or osteocalcin levels; during the Nordin tests, the fractional calcium excretion index (CaU/CrU) and the fractional hydroxyproline excretion index (OH Pro/CrU) remained unchanged. In contrast, the phosphorus load was followed by a decrease in the maximum phosphorus reabsorption threshold (TMP/DFG), which was more marked on the 5th day than on the 15th day; a slight fall in serum phosphorus levels was also seen on the 15th day. These findings suggest that phosphorus supplementation should be intermittent rather than continuous. Analysis of subjects aged 50 to 69 years failed to disclose any significant differences between patients with and without osteoporosis.