The mechanical properties of the human uterine cervix are determined mainly by the connective tissue component, whereas it is doubtful whether the scanty smooth muscle component is of any physiological importance. Histological and biochemical analyses reveal a fibrous connective tissue similar to that found in skin and tendon. Degradation of the collagen and an increase in some special dermatan sulphate proteoglycans can at least partly explain the pregnancy-associated softening of this connective tissue. Relatively high oestrogen levels seem to be an absolute condition for the process, even when it is induced pharmacologically. Treatment with progesterone-receptor blockers, PGE2, PGF2 alpha or relaxin in pregnancy induce cervical softening as well as histological and biochemical changes which cannot be distinguished from the physiological cervical softening which takes place in late pregnancy. Prostaglandins and relaxin might interact and could include cytokines such as interleukin-1 during the process. The effect of cervical tents cannot be explained only by the radial pressures they exert. Most probably stimulation of local prostaglandin synthesis is also involved.