Changes in serum and urine biochemical indices have been studied in ten normal subjects in the four hours following the ingestion of four proprietary calcium supplements. Each was taken in a dose containing 1 gram of elemental calcium. The four preparations were ranked according to the amount of calcium absorbed in the order Spar-Cal and Calcium Sandoz greater than Os-Cal greater than Ossopan. There were no significant differences between the four preparations in the changes in parathyroid hormone (PTH) and urine hydroxyproline levels. For this reason, the four results from each subject were averaged. Following the calcium load there was a reduction in mean PTH from 0.16 +/- 0.01 to 0.10 +/- 0.02 micrograms/l (p less than 0.001) and a decline in urine hydroxyproline/creatinine ratio from 20 +/- 1 to 17 +/- 1 (p less than 0.02), suggesting that bone resorption responds immediately to dietary calcium intake. There was a rise in urine sodium excretion which correlated with the indices of calcium absorption (r = 0.63, p less than 0.01) but not with the sodium content of the calcium preparations. This effect could be important, particularly in elderly patients on borderline sodium intakes.