Changes on facet joint articular cartilage biology after chymopapain- induced disc height loss were determined in adult mongrel dogs. Disc height decreased to 50% at 2 weeks and returned to 80% at 6 months. The biochemistry and histology of the involved-level facet joints were followed at 2 weeks, 6 weeks, 3 months and 6 months after the injection. X-ray study showed a narrowing of the joint space and overlap of the facet joints at 6 weeks and at 3 months. Some return toward normal appearance was noted at 6 months. There was a loss of Safranin-0 stain at 6 weeks, which persisted at 3 months, although some cells showed increased Safranin-0 staining in the surrounding matrix. This was further improved at 6 months. The water content was unchanged early, but had decreased significantly at 6 weeks. The hexuronic acid content, already dropping at 2 weeks, was significantly lower at 6 weeks, at 3 months, and at 6 months. However, by 6 months, it had increased, compared with the 3-month value. Synthesis of proteoglycan was depressed only at 6 weeks. Similar changes were found in the facet cartilage in the joints above and below the last injected disc level. Results of this study would suggest that there is a cause-and-effect relationship between disc pathology and facet pathology and this can affect adjacent disc facet-joint biology. The initial facet lesion described appears to be potentially reversible, but a long-term disc height decrease might be expected to cause irreversible osteoarthritic-like changes in the adjacent facet joint.