Abstract: A prosthetic nucleus pulposus for replacing the natural nucleus pulposus of an intervertebral disc. The prosthetic nucleus proposus comprises a partially collapsed sealed envelope formed from a material which is permeable to extracellular body fluid. The envelope contains a solute which provides an osmotic potential across the walls of the envelope. In use, the partially collapsed envelope is surgically implanted in the hallowed-out interior of an intervertebral disc and is allowed to absorb fluid, whereby expansion of the envelope and subsequent disc expansion is accomplished.