Hormonal changes around the parturition in rats

Concentrations of serum oxytocin, cortisol, progesterone and estradiol in pregnant rats were assayed by radioimmunoassay methods. The levels of these hormones increased in pregnancy. Around the parturition, oxytocin or estradiol did not show any change, cortisol increased, and progesterone decreased prominently. The trigger of initiation of labor was discussed.