In order to elucidate the increase in cervical dilatability and softening towards the end of gestation, the relationship between the changes in the extensibility and proteins, collagen contents and amino acid compositions of the uterine cervix in rats was studied. 1. Water-soluble proteins increased from the beginning of gestation to approximately third day of pregnancy, after then decreased gradually to the 20th day. Changes of total protein which was measured after solved by 1N NaOH was found to be similar to that of water-soluble proteins. On the other hand, 0.45M NaCl-soluble proteins increased gradually during gestation, whereas gel electrophoresis pattern of nonpregnant and 20th day of pregnancy showed the same pattern. This indicates the qualitative changes of cervical proteins during gestation. 2. In the cervix at 20th day of pregnancy, on the amino acid composition, Hydroxyproline values by which collagen was characterized were 69.5% of those in the nonpregnant cervix, whereas upon certain amino acids, Glycine and Glutamic acid, no significant change was observed. This indicates the decline of collagen and the increase of noncollagenous proteins. 3. Estriol caused some decrease in the water-soluble proteins and 1N NaOH-soluble total proteins of cervix of the 15th day pregnant rats, whereas increase in the 0.45M NaCl-soluble proteins, and on amino acid composition similar to that of 20th day pregnant rats. This indicates the stimulating effect of estriol on ripening of the uterine cervix in rats. 4. From these findings changes of proteins, collagen contents and amino acid composition of the uterine cervix in rats is generally similar to the changes of extensibility during gestation, thus it is assumed that the protein metabolism is intimately related with ripening of cervix uteri.