Radioimmunoassay of plasma and pituitary oxytocin in pregnant rats during various stages of pregnancy and parturition.

Radioimmunoassayable plasma and pituitary oxytocin (ROT) was measured during pregnancy and parturition. The highest mean plasma ROT level was seen in maternal rats on day 20 of pregnancy followed by a non significant decrease on day 21 reaching the basal level just before delivery. Plasma ROT level declined continuously up to the time of delivery of the first pup and then gradually increased until the completion of parturition. The highest maternal mean pituitary ROT level was found on day 21 pregnancy. Just before delivery of the first pup, the mean pituitary ROT level significantly declined by 22%. Pituitary ROT level declined steadily during the delivery of the first five pups. This pattern of reduction was reversed and ROT levels increased between the delivery of the fifth pup and the completion of labor. The increase of maternal pituitary and plasma ROT levels during delivery of the pups suggest that maternal ROT may play a role in normal parturition. Fetal hypothalamic-pituitary ROT does not seem to play any part in parturition.