The occurrence of haver sian remodeling has been measured in control and parathyroidectomized adult dogs given a tracer dose of calcium-45. Autoradiographs, microradiographs and histologic sections were made of samples of diaphyseal and metaphyseal bone and the numbers of resorption cavities and newly formed haversian systems compared for the control and experimental animals. The results indicate that haversian remodeling is reduced in the parathyroidectomized animals. Measurements of the actively calcifying lengths of the systems show that the reduction in the remodeling incidence in the experimental animals is not the result of a shorter length undergoing calcification but of a smaller number of systems being formed. Two of the dogs used in the experiment were females that had born and weaned a litter six weeks previously. In both these animals there is increased bone remodeling compared with the male dogs, however, the parathyroidectomized female shows a reduced incidence of remodeling compared with her female control.