In order to study the effect of angulation and asymmetric loading on the progression of spinal curvature a rat tail model was used to apply asymmetric loading across an isolated vertebra and to measure its growth response. Two 25 mm external ring fixators were glued to pins which had been inserted percutaneously into the 8th and 10th caudal vertebra of 6-week old Sprague-Dawley rats. Calibrated springs and 15° wedges mounted on stainless steel threaded rods passing through holes distributed around the rings imposed a 30° curvature and axially loaded the vertebrae in the instrumented tail region. Fluorochrome labels and radiographs were used to document the progression of vertebral wedging. Initially the wedging was entirely in the intervertebral discs, but by 6 weeks the discs and vertebrae were approximately equally wedged. Fluorochrome labeling confirmed that the vertebral wedging resulted from asymmetrical growth in the physes. The amount of longitudinal growth in these tails was similar to that occurring during human adolescent growth.