Circulating concentrations of oxytocin during the estrous cycle and early pregnancy in sheep

Plasma concentrations of oxytocin and progesterone have been measured by radioimmunoassay in jugular venous blood obtained daily from 5 sheep during 2 estrous cycles and in early pregnancy. Concentrations of oxytocin were relatively high (15-30) pg/ml) during the luteal phase of the cycle, but fell at estrus (to 1-17 pg/ml). A fall in oxytocin was also observed on day 15 of pregnancy, when, as expected, progesterone levels remained high. It is suggested that raised basal levels of oxytocin are unlikely to cause the increased uterine release of prostaglandin F2 alpha which occurs at the end of the estrous cycle.