The osteoclast number and its relation to parathyroid hormone have been studied in rat alveolar bone by quantitative histology and fluorescent labeling. The osteoclast number decreases 60 h after parathyroidectomy and remains constant for the next 132 h. Parathyroid hormone administration to parathyroidectomized animals 96 h after the operation induces an increase in osteoclast number within 12 h to somewhat above those of control animals. The elevated osteoclast counts remain constant for 60 h then rapidly fall over the next 24 h to the level seen in untreated parathyroidectomized animals. As determined by fluorescent labeling, normal alveolar bone resorption and formation were disturbed by parathyroidectomy, such that significant bone formation occurred for only 6 days after surgery, after which a quiescent state followed.