Partial discectomy was donein vitroon 15 human lumbar discs from donors 20–40 years of age. The change of disc height, radial disc bulge, and intradiscal pressure was measured in relation to the mass of central disc tissue excised. Disc height decreases and radial disc bulge increases approximately in proportion with the mass of the tissue excised. At the same time, intradiscal pressure decreases. On average, removal of 1 g of disc tissue results in a height decrease of 0.8 mm and a radial bulge increase of 0.2 mm. Removal of 3 g of central disc tissue lowers the intradiscal pressure to approximately 40% of its initial value. A mechanical model is presented which explains the observed relation between the volume of material removed and the subsequent change of height, bulge, and pressure. The experimental results and the model provide insight into the rationale of disc pressurization. A high intradiscal pressure is a prerequisite for the mechanical function of the disc under physiologic conditions.