Profiles of fixed charge density across young and old disc were measured by a modification of the tracer-cation method which prevented proteoglycan loss. The fixed charge density of the nucleus was 0.28 mequiv./g in the young disc and 0.18–0.24 mequiv./g in the old disc. The fixed charge density varied steeply across tha annulus, falling to about 0.07–0.13 mequiv./g in the outer annulus. Profiles of the partition coefficients for small ionic solutes were calculated using these values of fixed charge density, measured profiles of water content and the Gibbs-Donnan equilibrium equations. Measured values of the sulphate ion partition coefficient compared well with the calculated values.