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Summary Experiments have been discussed in which the specific activities of milk calcium and blood calcium have been compared after intravenous and oral doses of radioactive calcium administered to lactating dairy cows, along with investigations to determine the cause of the differences observed. After intravenous doses the specific activity curve of blood calcium as it declined passed through the peak of the milk calcium curve corrected to average time of secretion. After the peak, the milk specific activity usually remained higher than the blood but continued to decline at the same rate as blood. After oral doses wide variations in response have been observed. In some animals the specific activity of milk calcium has exceeded the highest observed specific activity of blood, but in others they were nearly equal at their maximum. After the peak the decline curves have been very similar to those after an intravenous dose. Cows that had previously developed milk fever were found to exhibit greater differences in specific activities of blood and milk calcium in early lactation than other cows; yet these differences were observed also in cows that were not highly susceptible to milk fever. No satisfactory explanation for the excessive proportion of radioactive calcium in milk has been discovered. Paper electrophoresis, arterial-venous differences, and experiments involving the constant injection of stable calcium failed to support the hypothesis that a high specific activity fraction of blood calcium existed which might be used preferentially for milk formation. Mass spectrographic analysis of blood and milk calcium of one cow showed no differences in proportions of six calcium isotopes, which indicated that the udder was not concentrating Ca 45 per se. The existence in the udder of a variable calcium reservoir from which the calcium is not readily exchanged with blood calcium, and which is used for milk formation, has been postulated to explain the differences in time between equal specific activity values of blood and milk calcium.