Non-pregnant and pregnant rats of known gestational age were killed at intervals and their uterine cervices were excised and digested with papain. Glycosaminoglycans thus extracted were separated by cellulose acetate electrophoresis and stained with Alcian Blue. Glycosaminoglycans were identified by comparison with standards and by serial degradation with chondroitin ABC lyase, butyl nitrite and leech hyaluronidase. Dermatan sulphate, hyaluronic acid and heparan sulphate were identified and quantitatively determined by densitometry. The overall concentration of glycosaminoglycans changed little during pregnancy. A 3-fold total increase in uronic acid paralleled the increase in cervical weight. Hyaluronate content, however, increased 17-fold, and rose from 6% of total glycosaminoglycans in the non-pregnant state to 33% at term. Furthermore, the ratio of hyaluronate to hydroxyproline increased 10-fold. These changes are consistent with an accumulation of hyaluronate in the interstices between collagen fibres, resulting in the softening of this tissue that is seen in late pregnancy.