Changes (mean ±SEM) in diameter of corpus luteum, plasma concentrations of progesterone (P4; n= 4) and estradiol-17β (E2; n= 4). The CL became visible by Day 4, reaching a plateau on Day 9.0±0.1 and luteolysis began by Day 12.2 ± 0.2 after the ovulation. The CL diameter was directly associated with the plasma progesterone concentration during the cycle (r = 0.93; p<0.05). Multiple low peaks of plasma E2 during the luteal phase (Days 1-14) and a preovulatory peak was observed during the follicular phase (Days 14-15). Arrow indicates the ovulation.