The 6-hour distribution of a tracer dose of Ca45 was studied in rats at various intervals after parathyroidectomy and sham operation. In young rats studied 1 day after parathyroidectomy there was a marked decrease in plasma Ca and a small decrease in the "6-hour exchangeable Ca." One week after parathyroidectomy there was no further decrease in plasma Ca but a significant further decrease in 6-hour exchangeable Ca. In old rats studied 1 week after parathyroidectomy there was a somewhat smaller decrease in plasma Ca concentration and no change in 6-hour exchangeable Ca. It is concluded that removal of the parathyroids does not affect the amount of bone Ca available for exchange directly, but may act indirectly in the following sequence: decreased parathyroid hormone[forward arrow] decreased bone resorption [forward arrow] decreased new bone formation [forward arrow] decreased exchangeable Ca.