The purpose of this work was to provide further knowledge about bone cell kinetics in the metaphysis of the growing long bone. Seventy rats were sacrificed from 1 to 120 h after injection of tritiated thymidine. Autoradiographs of 3 µm thick sections of the proximal tibial metaphysis were studied in a manner which allowed evaluation of labeled cell nuclei as a function of increasing age of metaphyseal tissue. A cell cycle duration study for osteoprogenitor cells was done. Labeled osteoprogenitor cells and osteoblasts first appeared at 1 h post-injection. The great majority of all labeled osteoprogenitor cells and osteoblasts was found within 1 mm of the growth cartilage-metaphyseal junction (GCMJ) at all times, apparently migrating with the moving GCMJ. In contrast, labeled osteoclast nuclei first appeared at 24 h post-injection within 0.3 mm of the GCMJ and remained always with the area of bone surface with which they were first associated, even as the GCMJ migrated away. By 5 days post-injection, the source of new labeled osteoclast nuclei in the metaphysis near the GCMJ was depleted, whereas that for the osteoblasts remained. The existence of two kinetically different, as well as ultrastructurally different, members of the metaphyseal osteoprogenitor cells population is postulated. A cell cycle time of 39±18 h was found for the osteoprogenitor cell population, but has limited meaning. A schema for metaphyseal bone cell movements during longitudinal bone growth is presented.