Fifteen minutes after intraperitoneal injection of sulfur-35 as sodium sulfate to 7-day-old rats the concentration of the isotope was highest in the cartilage at the epiphyseal-diaphyseal junction of long bones but was demonstrable throughout the entire epiphysis. Up to the 24th hour the pattern of deposition did not change as the concentration continued to increase.

As centers of secondary ossification developed, there occurred in them an increased concentration of some form of sulfur, insoluble as the barium salt. This sulfur was probably derived from the cartilage which the center of secondary ossification replaced.

Up to about the 30th minute after injection the sulfur-35 was deposited transitorily in a high concentration in discrete loci of the bone marrow. Excluding this transitory deposition, the highest concentration of the radioisotope in the bone marrow was seen in the specimens removed 24 hours after administration.

In the bone shaft the sulfur-35, in a form which was insoluble in an alkaline solution of barium ions, was deposited diffusely up to the 24th hour after its administration. Thereafter, the radioisotope decreased in concentration in the middle portion of the bone shaft. However, the concentration of a similarly characterized sulfur-35-bearing component in the ends of the diaphysis continued to increase up to at least the 96th hour after injection.

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