Abstract
SUMMARY: A study was made of the effects of both systemic and intrauterine progesterone and medroxyprogesterone acetate (MPA) administration on the duration of pregnancy in the guinea-pig. In no case was pregnancy prolonged beyond normal term. When progesterone and MPA were administered to non-pregnant guinea-pigs (in doses up to 50 times that required to block myometrial activity in the rabbit) there was no effect on the amplitude and frequency of intrauterine pressure cycles recorded in vivo by means of intrauterine balloons. The response of the uterus to a standard dose (20 m-u.) of oxytocin was unchanged by progesterone. The administration of progesterone by an intrauterine, instead of a systemic, route did not alter the result. It is concluded that progesterone is not a myometrial blocking agent in the guinea-pig.

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