In a series of thirteen sheep, experiments were carried out during the last month of pregnancy to assess the extent of progesterone influence on the myometrium. Staircase effects were recorded from the longitudinal fibres of the myometrium in the intact ewe and also in vitro, and the threshold dose of oxytocin required to elicit a contraction was determined. The results, both in vivo and in vitro, demonstrate that the progesterone influence declines before parturition and it is concluded that the extrusion of the foetus is accomplished by an oestrogendominated myometrium. This finding is discussed in relation to the fact that no fall in the progesterone level in blood can be detected before parturition. The oxytocin thresholds determined in vivo suggest that the sensitivity of the myometrium increases with the approach to parturition.
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