1. The uptake of inorganic pyrophosphate (PPi) from blood to bone was investigated in the rat in vivo.

2. PPi is taken up by the bone, where it appears both as PPi and as inorganic orthophosphate (Pi). The latter is due at least partly to local hydrolysis.

3. The fraction of injected PPi taken up by bone, measured as total Pi, was in the same range as that of technetium—tin—PPi, diphosphonates, technetium—tin—ethane-1-hydroxy-1,1-diphosphonate and Pi, but lower than that of calcium.

4. The plasma half-life of PPi is in the same order of magnitude as that of technetium—tin—PPi, diphosphonates, technetium—tin—ethane-1-hydroxy-1,1-diphosphonate, Pi and calcium.

5. PPi, diphosphonates and their technetium complexes are only partly ultrafiltrable in plasma.

6. It appears that the technetium complexes behave in a similar fashion to free PPi or diphosphonate.

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