Using a quantitative cytochemical method, explanted calvaria of 1- to 6-day-old male rats were used for the measurement of the in situ effect of parathyroid hormone (PTH) on the activity of alkaline phosphatase (AP) in the calcifying bone. The earliest effect of PTH was observed at 4--7 min. The direction and magnitude of the response of AP was dependent on the age of the animal and on the dose of PTH (50 fg-500 pg/ml). The calvaria of younger animals (2-days old) showed a decrease in AP activity, whereas in 5-day-old rats, AP activity was stimulated by PTH. In 5-day-old animals, the linear dependence of ATP stimulation on the PTH dose in the concentration range 50 fg-50 pg/ml could serve as the basis for a bioassay of a high sensitivity. The age-dependent response of AP to PTH may reflect the difference between the early and late stages of the calcification process. Our observations may reconcile conflicting reports on the effect of PTH on bone AP.