1. The effects of a multiple-segment posterior spine fusion of the modified Hibbs' type in pedigreed German shepherd puppies were studied during and at the completion of growth.

2. In the areas of spine where solid fusion was proved, the following changes were observed: reversal of the normal anterior concavity producing lordosis of varying degrees; anterior flaring of the vertebral bodies; narrowing and posterior wedging of the intervertebral spaces with anterior bulging of the discs; and retardation of longitudinal growth. The shortening of the portion of the spine operated on was the result of retardation of longitudinal growth, narrowing of the intervertebral spaces, and earlier closure of the vertebral epiphyseal plates.

3. These changes are apparently produced by the gradual development in the fusion mass of a posterior anchoring mechanism and are determined by the level of efficiency at which this mechanism operates during the period of vertebral growth.

4. At the interspaces corresponding to the major defects in the fusion mass, the kyphotic contour of the spine was maintained without any significant narrowing or posterior wedging of the intervertebral spaces.

5. The duration of growth in the dog was not long enough to permit study of the effects of a solid mature fusion on continued vertebral longitudinal growth, since maturity of the fusion mass occurred at or near the end of the normal growth period of the dog.