Progression of scoliosis deformity during growth is thought to be caused by asymmetrical loading, resulting in asymmetrical growth with vertebral and disc wedging in a "vicious cycle." The purpose of this study was to quantify the changes in disc thickness during growth in rat tails subjected to compression or distraction loading for 6 or 9 weeks, to investigate the hypothesis that disc growth is mechanically modulated. Six-week-old Sprague-Dawley rats were studied with compression loading (13 animals) or distraction loading (15 animals) applied to their tails, and there were 8 sham animals. Loading was applied to tail segments by means of an external ring fixator. Radiographic measurements of disc thickness were made at biweekly intervals. From the initial to final radiograph, compressed discs had reduced thickness averaging (+/-SD) 0.50 +/- 0.28 mm, distraction discs had average increased thickness of 0.20 +/- 0.42 mm, and sham discs lost an average of 0.21 +/- 0.18 mm of thickness (analysis of variance p < 0.001). There was an "initial change" in disc thickness averaging 0.18 +/- 0.32 mm in nonloaded discs, which was similar in magnitude to the elastic deformation and was attributed to disc swelling under anesthesia. These results indicate that growth in disc thickness is mechanically modulated by axial loading in growing rats.