The authors analyse the results obtained during 54 radioisotope investigations using 45Ca in 13 cases of idiopathic hypercalciuria, 12 cases of osteoporosis, 3 cases of Paget's disease, and 2 cases of osteomalacia including one of Fanconi's disease in an adult. In 12 patients, repetition of the radio-isotope test two, three or four times; permitted the authors to study the effects of the treatments administered: calcitonin, phosphate, vitamin D, parathormon, oestrogen. Calcitonin increases intestinal absorption and reduces bone reabsorption and also accretion. Phosphate greatly increases accretion and bone reabsorption in vitamin-resistant osteomalacia of adults. The synthetic fragment 1--34 of human parathormone increases accretion and reabsorption but does not modify the calcium balance. The addition of estrogen reduces reabsorption and slightly increases accretion in two osteoporotic patients producing a positive calcium balance. This method of investigation is of great interest to assess the effects of a drug on calcium metabolism and on the two processes of bone remodelling.