1. Dialysed extracts of rat costal cartilage were shown to possess an enzyme that hydrolyses inorganic pyrophosphate. 2. Inorganic pyrophosphatase activity assayed in the presence of 2mm substrate was maximal at pH6·8. 3. Mg2+ was essential for activity, which was greatest with 10mm or higher concentrations of Mg2+. 4. Extracts prepared from cartilage taken from suckling rats (<20g.) showed little or no hydrolytic activity, but as rat weight increased inorganic pyrophosphatase activity was detected, increased to a maximum in tissue from animals weighing about 40g., and then rapidly declined. 5. The increase in inorganic pyrophosphatase activity was associated with an increase in the uptake of 45Ca by the cartilage in vivo. 6. Accumulation of calcium, inorganic phosphate and magnesium occurred when inorganic pyrophosphatase activity was at its maximum. 7. Alkaline phosphatase activity, measured in the same extracts used to determine pyrophosphatase activity, was highest in the tissues of the animals weighing <20g., and decreased as inorganic pyrophosphatase activity increased to its maximum. 8. There was no direct relationship between alkaline phosphatase activity and the onset of calcification.

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