Growth and remodelling of the first cervical vertebra were studied in the rat by means of biometry, vital staining (alizarin red S and oxytetracycline) and histology. The measurements showed a change in the ratio of the dorso-ventral to the transversal diameter in the lumen after obliteration of the dorsal synchondrosis.The pattern of labelling between the three segments of the vertebrae joined by synchondroses in the young animals indicated that expansion of the vertebral lumen took the form of a displacement of all three parts making up the bony ring. After closure of the dorsal synchondrosis the two ventral ones were still active and the lumen size increased more in the dorso-ventral direction than transversally. Alizarin red S and oxytetracycline tended to persist in the ventral part of the vertebra, while the staining almost disappeared from the dorso-lateral segment. Resting lines were found laterally to the ventral synchondroses in haematoxylin and eosin stained sections, but not in the ventral segment between the cartilages.The growth of the rat atlas is the result of an early rapid cartilage-mediated expansion of the vertebral lumen in conjunction with the growth of the spinal cord and at a later age mainly of a displacement of the ventral vertebral segment leading to the final form of the vertebra and its lumen.