For patients who require treatment over a period of some years, intranasal administration of synthetic salmon calcitonin (SSCT) obviates the discomfort associated with administration by injection. Moreover, this mode of administration is not associated with the side effects normally encountered when calcitonin is injected intramuscularly or subcutaneously. The aim of this study was to assess, in normal subjects, the biological activity of nasal SSCT by comparing the fluctuations of parameters reflecting calcium-phosphorus metabolism after nasal instillation, injection of SSCT and injection of placebo, respectively. In nine healthy subjects, this instillation of 200 IU of SSCT into the nasal cavity caused a fall in serum calcium, a fall in serum phosphorus and a transient rise in parathyroid hormone levels similar to that observed after the intramuscular (i.m.) injection of 80 IU of SSCT. SSCT whether administered by the nasal route or by injection, does not inhibit endogenous calcitonin secretion. There were no changes in serum beta-endorphin, magnesium or erythrocyte magnesium levels after administration of calcitonin by the intranasal route or by injection.