SUMMARY: The effect of calcitonin on the incorporation of [3H]proline into bone hydroxyproline was investigated in young growing rats. The action of the hormone was studied under the following conditions: (1) acutely, in both intact and thyroparathyroidectomized rats, (2) after 20 days of daily treatment in both intact and thyroparathyroidectomized animals, (3) in thyroparathyroidectomized animals treated with parathyroid extract for 20 days, and (4) in animals fed a low calcium diet for 25 days. Calcitonin had no effect when given either acutely or long-term to intact animals. [3H]Proline incorporation into bone hydroxyproline was increased in rats fed low calcium diet, in rats treated with parathyroid extract for 20 days, and in acutely thyroparathyroidectomized rats. In each instance, calcitonin prevented or reduced this increase. The results are interpreted as evidence that, with the doses used, hydroxyproline synthesis under normal conditions is unaffected by the hormone. However, where hydroxyproline formation was increased, calcitonin reduced this increase.