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Although clinical instability is an in vivo problem, most spinal instability criteria are either subjective or are based on in vitro experiments. The authors performed an in vivo experiment using a canine model to study the natural history of spinal instability as a function of healing time up to 12 weeks. Three injuries were produced surgically: sham; laminectomy at C4; and bilateral facetectomy at C4-C5. Three 1.5-mm steel balls were implanted into C3 to C6 vertebrae at the time of surgery. Standardized functional flexion-extension stereoradiographs of the cervical spine were obtained before injury, immediately after injury and at 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7.5, 9, and 12 weeks postinjury and immediately after killing the animals. In general, the authors found decreased ranges of motion (ROM) at the C4-C5 level, compared with the pre-injury values, for all injuries, but most significantly for the facetectomy. The maximum decrease occurred between 0 and 0.5 weeks postinjury. Between 2 and 12 weeks, there was recovery in the ROM, especially for the two less severe injuries. The changes in the ROM at each spinal level were explained by simultaneous presence of a destabilizing factor, caused by the three different injuries with the sham as the least and the facetectomy as the most destabilizing injury, and a stabilizing mechanism of muscle spasm in the beginning and of healing and other adaptive responses in the late phase after injury. Because of the significant differences between the canine model and the human cervical spine, the present findings should be extrapolated to the human situation with caution.