1. Measurements have been made of the relative calcification of different types of bone in tibia of the rabbit at the ages of six weeks, three and a half months and seven months by comparing their absorption of x-rays.

2. Calcified cartilage is between 8 and 10 per cet more highly calcified than periosteal and endosteal bone and about 20 per cent more highly calcified than bone formed immediately adjacent to cartilage.

3. Young and adult bones have a framework of approximately the same strength; that is, calcified cartilage, bone adjacent to cartilage and the interstitial areas of periosteal and endosteal bone have each approximately the same degree of calcification at all ages.

4. Adult rabbit bone approaches uniform calcification throughout, equal to the calcification of the interstitial areas of periosteal and endosteal bone. Evidence for this is the replacement of the lowly calcified epiphysial bone by osteones of higher calcification.

1. The detailed anatomy and calcification of the upper half of the tibia in rabbits varying in age from six weeks to twelve months has been studied.

2. The structure of the bone varies at different levels, but a section taken from the same level in the tibia from animals of the same age presents a reasonably constant picture.

3. It has been shown that this variation in structure at different levels is directly related to a difference between the axis of growth and the bone axis. This difference is a result of the unique shape of the tibia.

4. Autoradiographic studies confirm the localised concentration of radioactive strontium in areas of active bone formation where uptake is rapid.

5. The long retention of radioactive strontium in the skeleton (that is, the slow turnover) is a result of the slowness of resorption of bone (endosteal, periosteal or Haversian) in the cortex. Not only is the process slow but it is extremely localised.

6. The significance of these anatomical and physiological characteristics in relation to radiation injury is discussed.