Total serum calcium of normal men, a hypoparathyroid man, two thyroparathyroidectomized men, and intact and thyroparathyroidectomized dogs were studied at multiple intervals following the acute administration of synthetic salmon calcitonin. Calcitonin produced marked fluctuations in serum calcium in one normal man and a biphasic hypocalcaemic response in another. In four of five intact dogs, calcitonin caused absolute or relative hypercalcaemia. In contrast, administration of calcitonin to thyroparathyroidectomized dogs caused a hypocalcaemia with less fluctuations and with no periods of hypercalcaemia. It is possible that some of the paradoxical responses of serum calcium induced by exogenous calcitonin are due to overcompensation by parathyroid hormone. However, other mechanisms may be involved. Our findings indicate that when the disturbing influence is sufficiently great, the control of serum calcium is not as well modulated as previously suspected. In addition, our findings of paradoxically hypercalcaemic responses to calcitonin indicates that the pathophysiologic interpretation of serum calcium at any single moment in time following the administration of this hormone to either intact or thyroparathyroidectomized men or dogs is a precarious endeavour.