It has been suggested that the intensity of hypocalcaemic response to the administration of calcitonin may reflect the degree of osteoclastic activity and thus the state of the parathyroid function. We have performed the calcitonin test in 14 healthy volunteers and in 17 patients with chronic renal failure undergoing haemodialysis, The test involved the intramuscular injection of 100 MRC units of synthetic salmon calcitonin, serum calcium being determined before and after 5 h of the injections of the hormone. In 7 of the patients, the test was repeated using intranasal administration by means of a spray of calcitonin. In renal patients, serum calcium fell from 9.73 ± 0.76 to 7.88 ± 1.01 mg/dl (1.85 ± 0.67 mg/dl). In the control group, serum calcium fell from 9.51 ± 0.56 to 8.89 ± 0.34 mg/dl (0.62 ± 0.35 mg/dl). The calcium fall in renal patients was significantly greater than in the control group (p < 0.001). In patients there was a statistically significant correlation between the fall in serum calcium and the level of alkaline phosphatase, but no with the serum concentration of immunoreactive parathormone (C-Terminal). After intranasal calcitonin, serum calcium fell from 9.81 ± 0.55 to 8.83 ± 0.72 mg/dl (1.42 ± 0.42 mg/dl), a reduction comparable to that obtained after the intramuscular injection of the hormone in the same patients (1.84 ± 0.69 mg/dl). These results provide evidence of an increase in osteoclastic activity in chronic renal failure, perhaps reflecting parathyroid hyperfunction. The lack of correlation between serum calcium fall and the levels of parathormone probably means that there is an inadequancy of the C-terminal radioimmunoassay of this hormone in renal failure. The calcitonin test may be an usuful index of bone state in uraemia, in addition to alkaline phosphatase, osteocalcin, and immunoreactive parathormone. The intranasal route has been effective, and it should make the test easier to perform; however, it is still in an experimental phase.