Effects of decompressive procedures or dorsal injuries on flexion-extension, 4-point bending properties of the L-3 and L-4 motion segment in dogs were quantitated and compared. Hemilaminectomy did not significantly (P less than 0.05) affect mechanical properties of the spine. Bilateral facetectomy caused a small increase in the range of motion and a 56% decrease in ultimate bending strength. Excision of the supraspinous and interspinous ligaments decreased the flexural stiffness of the spine, increased the range of motion of the interspace, and decreased ultimate flexion bending strength 62%. Dorsal laminectomy caused a marked decrease in spine stiffness in all phases of flexion and extension, increased the range of motion, and decreased ultimate flexion bending strength 75%. Seemingly, a hemilaminectomy was preferable to the dorsal laminectomy if adequate decompression and exploration were achieved with the hemilaminectomy.