The change of the contractility of rat's uterine muscle was studied in connection with the process of pregnancy and the following results were obtained. 1) Single sucrose gap method study revealed that the spike potentials were dominant in longitudinal muscles, while slow potentials were dominant in circular muscles, but in the terminal stage of pregnancy, the electrical activity in circular muscles became similar to that in longitudinal muscles. 2) In the middle stage of pregnancy, the intrauterine pressure is affected by the longitudinal muscles, while contraction of circular muscle is reflected on the small frequent change of the intrauterine pressure. 3) As the labor approaches, the coordination of contractions improves and the propagation of contractions from tubal site to the cervical end becomes smoothened. 4) The in vitro observation of delivery revealed that the fetal expulsion took place in the absence of cervical resistance, while the fetuses could not be expelled when the cervical resistance was present. This phenomenon confirmed the important role of cervix in labor. When the cervical resistance was absent, the delivery in the terminal stage of pregnancy could be more easily completed than in the middle stage, suggesting the contractility is more effective in the terminal. 5) In conclusion, the uterine contractility improves in the terminal stage of pregnancy, but the mechanism of labor can not be explained on the sole ground of uterine contractility, and other factors, such as cervical conditions, should also be taken into consideration.