Long-term ingestion of 100 p.p.m. of fluoride as NaF in the drinking water of the intact growing rat resulted in a slight but significant decrease of fasting plasma calcium concentration when compared with that in controls given distilled water. The plasma calcium response of both groups of rats to administered calcitonin was similar. The effect of parathormone on the plasma calcium response of thyroparathyroidectomized rats given NaF or distilled water showed a similar degree of hypercalcemia. Intact rats receiving fluoride or distilled water, given the same amount of parathormone, exhibited only a slight elevation of plasma calcium which was similar in both groups. It is concluded that the long-term ingestion of fluoride probably does not alter the response of the growing rat to endogenous or exogenous parathormone or calcitonin.