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Evidence that a humoral factor possessing relaxin-like activity is responsible for uterine quiescence in the late pregnant rat

Summary. When post-partum ovariectomized rats were linked to 21-day-pregnant rats by a cross-circulation system their myometrial activity was almost completely inhibited. No reduction in activity was induced by cross-circulation with other post-partum rats, or with rats in which myometrial activity had been suppressed with oestrogen. Changes in uterine activity could not be correlated with alterations in arterial blood pressure occurring during the experiments. Uteri rendered inactive by cross-circulation were found to be responsive to oxytocin. It is concluded that a humoral myometrial inhibitor is present in late pregnancy in rat blood and the possibility that it is relaxin is discussed.