Metabolic turnover of collagen in the developing molar periodontal ligament was studied in 10, 12 and 16 days old mice by autoradiography after the administration of [3H]proline. Grain counts, in number per unit area, were made over three different zones (apical, middle and cervical) of the sections and activity time curves were drawn. Calculated regressions (log10 grain density V time) were fitted and analyses of variance to test the linearity of the regressions were carried out using the computer. Slopes +/- S.E. of the regressions, and half-lives were estimated. This study has demonstrated that collagen turnover occurred throughout the whole thickness of the ligament, from bone to cementum. Therefore, there was no evidence of a metabolically active 'intermediate plexus' in the periodontal ligament. However, the turnover rates in the apical zones of the three groups studied were significantly higher than those in the middle zones, which in turn were higher than those in the cervical zones, suggesting a differential rate of collagen turnover within the periodontal ligament. This whole sequence was significantly higher in the 12 days old compared with the 10 and 16 days old groups. Estimated half-lives confirmed this pattern of rate of collagen turnover. The molar periodontal ligament behaves and functions as a 'stimulated system' during the period 10 to 16 days, with a statistically significant rate of collagen turnover at the twelfth day, which is also the time of tooth eruption in the mouse. Thus, collagen turnover is an essential and accompanying process of tooth eruption.