1. The effects are described of the calcium content of diets with a calcium: phosphate ratio of 1 fed to rats during the preceding 6 weeks on aspects of calcium and phosphate metabolism during the seventh week. The apparent absorption of calcium and phosphate, the urinary excretion of calcium and phosphate, the amounts of calcium and phosphate in the carcass and the fasting serum calcium were studied.2. Apparent absorption of calcium and phosphate decreases the longer an animal is on a high calcium diet and the more calcium there is in that diet. Under the same conditions urinary excretion of calcium is increased while urinary excretion of phosphate remains constant.3. The percentage of calcium and phosphate in the carcass is greater the longer an animal is on a high calcium diet and the more calcium there is in the diet, but dietary changes do not affect the serum calcium.4. Apparent absorption of calcium is not correlated with serum calcium but is significantly correlated with percent carcass calcium.5. A distinction is drawn between the effect of the diet currently being consumed (kinetic effect) and the effect of the diet consumed previously (adaptation).6. Changes in apparent absorption of calcium have a greater effect on calcium retention than on changes in urinary excretion.7. The mechanism bringing about the changes in absorption is discussed and it is concluded that serum calcium is unlikely to be the monitored parameter which detects the need for greater absorption; parathyroid hormone is probably not involved.
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