The molecular mobility of the chain dynamics of collagen was investigated by the thermally stimulated creep method on rat tail tendon after oral administration of cadmium (8 mg.kg-1.day-1) for six weeks. The high resolving power of the technique shows two manifestations of the pseudolathyrogen effect of cadmium: the polar side-chains of collagen, mobile in the immature specimen, which are cross-linked and so immobile in the mature specimen, remain mobile in the cadmium-treated mature specimen. There is also a subsequent decrease in the number of water molecules linked by two hydrogen atoms bound to the tropocollagen molecules. Probably these molecular modifications inhibit mineralization of the organic matrix and so osteogenesis.