Ovarian Sources of preovulatory progesterone peak in the rat

The content of progesterone and oestradiol was estimated in three generations of corpora lutea (CL) isolated from rat ovaries at 11.00 and 18.00 h at consecutive stages of 4-day oestrous cycle. The highest amount of progesterone was found in the youngest CL on the day of metoestrus and on the morning of dioestrus. The second, much lower rise was observed in this generation on the evening of pro-oestrus. The progesterone concentration was relatively high in the older CL at oestrus but decreased thereafter. The oldest CL contained very little of progesterone. All generations of CL contained very low amounts of oestradiol. These results suggest that not only the preovulatory follicles but also the youngest CL can contribute to progesterone peak before the ovulation in rats. As judged from CL progesterone content, morphological analysis, CL weight and size, full luteolysis of CL takes place when they are about 6 days old.