The effect of parathyroid hormone (PTH) on the spongiosa of the proximal tibia and the alveolar bone in immature rats was studied using a time marker by the injection of lead acetate. 1) When intact rats were fed on a low calcium diet, the promotion of the apposition of the alveolar bone and the longitudinal formation as well as the resorption of the spongiosa were observed, but no change was detected in the serum calcium level. The resorption was more remarkable than the formation. 2) By parathyroidectomy (PTX) or thyroparathyroidectomy (TPTX) the effect described in 1) disappeared completely, but there subsequently occurred a fall of the serum calcium level and a marked inhibition of the formation and resorption. The decrease in the appositional formation was stronger than in the longitudinal formation. 3) When PTH was injected into the rats having undergone PTX or TPTX, all of the effects in 2) were reversed with the recovery of the apposition being at an extremely high rate. All the foregoing results indicate that PTH has an evident anabolic action in addition to the action of increasing the bone resorption and that the sensitivity to PTH is stronger in the periosteal bone than in the spongiosa.