We studied the effects of mechanical and osmotic compression on sulphate incorporation into glycosaminoglycans of human femoral head cartilage. We found that both mechanical and osmotic compression produce the same lowering of sulphate uptake relative to uncompressed controls. It appears that this effect is not associated with changes in solute transport or changes in solute concentration in the matrix, but is due, in part at least, to an increased osmotic pressure acting on the chondrocytes. A second mechanism of action might be involved directly through the increased proteoglycan concentration in the pericellular environment, resulting from a reduction in the water content. We also found that glycosaminoglycan synthesis returned to its control level when the conditions prevailing in the matrix, in the absence of pressure or added solute, were restored.