1. In rabbits with chronic uterine fistula, estrogen was found to enhance oxytocin sensitivity of uterine muscle, and progesterone conversely to depress it.
2. The relation between automatic uterine movement and oxytocin sensitivity was pursued with rats by the Magnus method, and the following results were obtained : (a) Uterine movement of non-pregnant rats was most rhythmical in oestrus with high oxytocin sensitivity; (b) in early pregnancy, oxytocin sensitivity of the uterus temporarily lowered, but gradually rose from the middle through the late pregnancy, attaining the maximum in the time of labor; (c) when estrogen was administered in late pregnancy, oxytocin sensitivity was raised to such chdegree as in labor; (d) when the animals were pretreated with progesterone, labor was not induced even after the time of the expected period, and the administratione of estrogen could not immediately produce such particular oxytocin sensitivity as in labor; (e) this particular enhancement of oxytocin sensitivity in labor is considered to have important bearing on the induction of labor, and estrogen is assumed to act as a cause of this enhancement.