1. Posterior fusion of the thoracic spine in growing dogs produces both gross and histological alterations in the fused segments.
2. The gross changes include the production of lordosis in a normally straight or kyphotic canine spine and decreased growth in the over-all length of the fused area.
3. The microscopic changes include progressive maturation of the bone in the fusion mass, narrowing of the discs spanned by the graft with associated increased tissue density and decreased cellular activity within the disc, and slight alterations in the arrangement of the cell columns in the involved epiphyseal growth cartilages.