Oestrogen peaks at about 11 am on the day of pro-oestrus. Between then and midnight there is a surge in progesterone, luteinizing hormone and follicle-stimulating hormone, and ovulation occurs at about 4 am on the next oestrus day. The following day, metoestrus, is called early dioestrus or dioestrus I. During this day, the corpora lutea grow to a maximal volume, achieved within 24 hours of ovulation. They remain at that size for three days, halve in size before the metoestrus of the next cycle and then shrink abruptly before oestrus of the cycle after that. Thus the ovaries of cycling rats contain three different sets of corpora lutea at different phases of development.