Calcitonin deficiency was produced in lactating and age-matched nonmated rats by thyroidectomy (TX) after transplantation of the parathyroid glands to a thigh muscle. At the end of lactation and a comparable period in the nonlactating rats, this condition resulted in femurs, vertebrae, and tibiae that weighed less than those in the thyroid-intact controls. Furthermore, the femurs of the CT-deficient rats were narrower at midshaft and shorter, indicating reduced bone growth. The reduction in bone mass in CT-deficient rats, although highly significant, was much smaller than that caused by lactation. Adequate thyroid hormone replacement therapy was provided by giving all the TX rats L-thyroxine (T4) sc or in the drinking water. The body weights of the lactating rats were heavier than those of their nonmated controls but TX had no significant effect on the mean body weight of either group. The previously observed lower concentration of serum calcium in lactating rats than in nonlactating thyroid-intact rats was also seen in TX rats, indicating that CT is not responsible for the relatively low serum calcium during lactation. Our results showing that the bones of TX rats (with T4 replacement) were smaller and lighter than those from thyroid-intact controls whether lactating or not do not support the concept that CT has a special physiological function to protect the skeleton during lactation.