Connective tissue in biopsy specimens taken from the lower part of the uterine cervix in 40 pregnant women at various gestational ages was compared to that in similar biopsy specimens from 15 nonpregnant women. The concentrations of collagen, sulfated glycosaminoglycans, and hyaluronic acid decreased during pregnancy. At the gestational age of 10 weeks, the collagen concentration was 70%, and at term 30%, of that in the nonpregnant cervix. After delivery, no further decrease was observed. The extractability of collagen increased during pregnancy, as well as during labor. Also, the water concentration increased. An increase in the collagenolytic activity was observed with advancing gestational age. The 2,4-dinitrophenyl-Pro-Gln-Gly-Ile-Ala-Gly-Gin-D-Arg hydrolytic activity (collagenase) and the concentration of leukocyte elastase increased gradually by a factor of 10. The physiologic importance of the collagen was also demonstrated, since the cervical dilatation time during spontaneous labor was long in women with high concentrations of collagen and short in women with low concentrations of collagen.