STUDY DESIGN: Pain behavior and immunohistological analysis in intervertebral disc (IVD) injury model. OBJECTIVE: To investigate pain behavior in a rat model of IVD injury using the CatWalk system. SUMMARY OF BACKGROUND DATA: There are few reports examining low back pain behavior in animal models. The CatWalk is a computer-assisted gait analysis system that provides an automated way to assess gait function and pain-related alterations of this behavior. METHODS: In the IVD injury group, L5-L6 IVDs were injured with a 24-gauge needle. Simultaneously, the neurotracer Fluoro-Gold (FG; Fluorochrome, Denver, CO) was injected into the L5-L6 IVDs. In the sham group, FG was injected into the L5-L6 IVDs only. Animals in the control group received no operation. One, 2, 3, and 4 weeks after surgery, the gait of rats in the 3 groups was investigated using the CatWalk system. One, 2, and 4 weeks after surgery, in IVD injury and sham groups, dorsal root ganglions from the L1 to L6 levels were resected. Dorsal root ganglions were immunostained for calcitonin gene-related peptide. RESULTS: In the IVD injury group, the mean stands of hind paws and the mean duty cycle of front paws at some time points were significantly higher than those in the sham group. Furthermore, the mean stride length of the front and hind paws and the mean swing speed of the front and hind paws at some time points were significantly shorter than those in the sham group. The proportion of calcitonin gene-related peptide-immunoreactive, FG-labeled neurons among all FG-labeled dorsal root ganglion neurons in the IVD injury group was significantly higher than the corresponding proportion in the sham group. CONCLUSION: These results suggest that IVD injury produced significant changes in rat gait, including longer stance phases and shorter strides. In the future, we may be able to apply the CatWalk system to the evaluation of behavior associated with pain in models of low back pain. LEVEL OF EVIDENCE: N/A.