Determinations of calcium and inorganic phosphorus were made on the blood of 5 groups of normal animals over periods of 4 to 8 months. The material included animals from recently acquired stocks as well as animals that had been living under laboratory conditions for long periods of time. The results were analyzed with especial reference to the occurrence of periodic and progressive variations in the absolute amounts of calcium and inorganic phosphorus and the relations between the two substances. It was found that, among animals living in the laboratory, both calcium and inorganic phosphorus, as well as all expressions of the equilibrium between the two substances, exhibited a definite tendency to a progressive increase or decrease) as the case might be, and that clearly defined periodic variations occurred in all classes of animals. The progressive change was most marked in the case of inorganic phosphorus, the periodic change in the case of calcium, while both conditions were dearly shown by various expressions of the relation between calcium and inorganic phosphorus.