When the published data on the radial disc bulge in relation to the axial compression of the motion segment are compared to a simple mechanical model, it follows that an axial inward bulge of the vertebral endplates should occur during compression. The model predicts that the disc height at its center should remain practically constant under compression. The axial endplate bulge has been measured in specimens of the human lumbar spine by stereoroentgen-photogrammetric methods. The results confirm the prediction that the axial endplate bulge is comparable in magnitude to the linear compression of the motion segment. Axial endplate bulge and deformation of the underlying trabecular bone are thus important determinants for the compression characteristics of the human spine.