1. Rats were given a rachitogenic diet for 28 days; some were dosed with vitamin D 2 or 3 days before they were killed and some received vitamin D throughout the experiment.
2. Parathyroid leucine aminopeptidase was measured histochemically as an index of parathyroid activity and silver impregnation as a criterion of bone calcification.
3. Calcium and phosphorus were estimated in bone ash and in serum. In rats killed 2 days after a single dose of vitamin D, increases were found in the Ca and P content of serum and bones and in epiphyseal Ca deposition. Parathyroid activity was decreased. In rats killed 3 days after a dose of vitamin D there was a marked increase in serum Ca and P but a less clear change in bone Ca and P although epiphyseal calcification was evident. The parathyroid activity was less than in rats dosed throughout the experiment.