The tails of growing rats were axially loaded in compression or distraction to investigate the hypothesis that vertebral and disc wedging during growth in spinal deformities is mechanically modulated. Twenty-eight 6-week-old Sprague-Dawley rats were assigned to one of three groups: compression loading, distraction loading, or sham (apparatus applied without loading). Two 0.7mm diameter stainless steel percutaneous pins were passed through two vertebrae and glued to 25mm diameter external ring fixators. Calibrated springs were installed on three threaded rods passing through the rings and compressed with nuts to apply compression or distraction forces between 25% and 75% of bodyweight. Radiographs made at weekly intervals for nine weeks were digitized to measure vertebral and disc growth rates. Loaded vertebrae grew at 68% of control rate for compressed vertebrae, and at 114% for distracted vertebrae (p<0.01 by ANOVA). Rate of change of disc thickness during the experiments was negative for all groups. However, comparison of radiographs before application of load with those made after sacrifice and removal of apparatus showed that compressed discs had reduced thickness averaging 0.49mm, distraction discs had average increased thickness of 0.13mm (p<0.05), and sham discs lost an average of 0.27mm of thickness. Results are compatible with the hypothesis that asymmetric loading of vertebrae and discs promotes scoliosis progression during growth.