Summary.1. The phosphorus turnover of the pineal has been studied with the aid of radioactive phosphorus in rats of different ages and compared with the turnovers of the adenohypophysis, neurohypophysis, tuber cinereum and cerebellum. The activities in these organs with exception of cerebellum are lowest during the first 10 weeks of the life of an animal. In the fertile age the activity remains at a constant high level and with exception of the neurohypophysis increases somewhat during senility. The activity of the cerebellum remains unchanged.2. Of the 42 organs and parts of the brain investigated, the phosphate metabolism decreased only in the ovaries after epiphysectomy to a statistically probable extent. Considerable increases in the turnover were also observed in the adenohypophyss, neurohypophysis and tuber cinereum in females.3. After castration an increased phosphorus turnover was obtained in the pineal body and the adenohypophysis. This increase was most pronounced in females.4. It appears to follow from these results that the pineal exerts an influence, directly or indirectly, especially upon the ovaries. There appear to be reciprocal effects of a similar nature between the ovary and the epiphysis as between the adenohypophysis and the ovaries.We wish to express our thanks to Professor Manne Siegbahn for his kindness in placing radioactive phosphorus at our disposal.