The decidual cell reaction (DCR) of ovariectomized mice treated with hormones and stimulated by the intrauterine injection of oil was investigated to obtain information about the control of uterine sensitivity during implantation. Three factors were studied; oestrogen given at the time of oestrus, progesterone and oestrogen given at the time of the nidatory surge.For the induction of the DCR by oil both 'oestrous' oestrogen and 'nidatory surge' oestrogen were essential, whereas neither were necessary for the traumatic DCR. The quantity of nidatory surge oestrogen was very critical; 0\m=.\01 \g=m\g. were effective, 0\m=.\0625 \g=m\g.inhibitory. There was no quantitative interaction between nidatory surge oestrogen and progesterone, indicating that in this situation the two hormones are acting independently. The optimum time interval between initiating the oestrogen surge and injecting the oil was between 4 and 8 hr.No response was obtained when the oestrogen surge was produced and the stimulus applied on the second day of treatment with progesterone; maximal responses were obtained on the fourth and fifth day and a reduced response on the seventh day. After an oestrogen surge on the fourth day, it was not possible to elicit an oil DCR to a further dose of oestrogen on the fifth day, indicating that the period of sensitivity induced by the surge on the fourth day is followed by a period of refractoriness.

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