1. A series of contraction in Krebs solution by repeated electrical stimulation to the progesterone-dominated rabbit uterus in vitro, was named as the transitional phenomenon and its mechanism was investigated. 2. The phenomenon was classified into 4 types and type A is taken as to be the. most typical. 3. The phenomenon can be devided into three stages and contractions of the 1st and 2nd stage were different in shape from the 3rd stage. 4. The phenomenon was affected by preservation of the muscle and finally disappeared. 5. Uterine horn showed the most typical phenomenon to the rest of the uterus and could hold it for a longer period of time. 6. The phenomenon was observed only once in a same strip of uterus. 7. The temperature of Krebs solution in experiment was the most suitable at 28°C to obtain it. Rising the temperature to 37°C obscured its appearance. 8. The optimal voltage for the phenomenon was 2 V/cm and 8 V/cm or more disappeared it. 9. The phenomenon was seen in both of longitudinal and transverse applications of electrical stimulation. 10. The phenomenon could not be observed when the muscle was depolarized below the certain level by excess K ion. 11. The phenomenon was least sensitive to the alteration of Ca ion in Krebs solution. 12. Hypertonic and hypotonic Krebs solutions disappeared it. 13. The phenomenon was closely dependent on interval and duration of the electrical stimulation. 14. From these data, the main cause of the transitional phenomenon is speculated as the shift of polarization of cell membrane. But decrement of both metabolism and conduction property in muscle cell by progesterone also ought to be considered.