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1. Adaptation to different dietary levels of calcium was produced by feeding a low (0-2%) calcium diet or one of two high (1-6 or 0-8%) calcium diets for 4 or 6 weeks. Adaptive changes in true and apparent absorption of calcium, apparent absorption of phosphate and urinary excretion of calcium, were observed. 2. Parathyroidectomy performed prior to adaptation did not greatly impair the ability of rats to adapt to different levels of calcium in the diet. The response of the rats to parathyroidectomy was affected by their subsequent dietary history. 3. Six weeks after parathyroidectomy the plasma calcium was significantly higher than it had been immediately post-operatively. This rise in plasma calcium was seen in the rats adapted to the 1-6% calcium diet but not in those adapted to the 0-2% calcium diet. 4. Parathyroidectomy performed after adaptation had taken place did not abolish the adaptive changes. The response of rats to parathyroidectomy was affected by their previous dietary history. 5. Large doses of cholecalciferol given for 8 days after adaptation had taken place increased the absorption of calcium in rats adapted to the 0-8% calcium diet thereby abolishing or reducing the adaptive differences in absorption between these rats and rats adapted to the 0-2% calcium diet. The cholecalciferol increased urinary calcium excretion but did not abolish adaptive differences in urinary excretion of calcium between rats adapted to diets with different calcium levels. 6. It is concluded that parathyroid hormone does not play a major role in mediating adaptation to different dietary intakes of calcium. The possible role of 1-25 dihydroxycholecalciferol is discussed.