1. The pattern of tritiated thymidine labelling in the cells of the epiphysial cartilage and metaphysis of the tibia in the rat is described for intervals of one hour to twenty-eight days after injection.

2. The region of dividing cells is defined and evidence given for a zone of reserve cells at the top of the cartilage columns.

3. The difficulties of quantitative grain count studies are discussed, and some approximate values are given for the generation time and mitotic cycle periods of the cartilage plate cells.

4. Some further evidence is given about the life cycles of the osteoblast and the osteoclast.