The development of ovarian follicular and luteal structures during the estrous cycle and early pregnancy in ewes was examined. Ewes were treated with prostaglandin F" to induce estrus; they were placed with either two fertile or two vasectomized rams and observed for estrus twice daily. Beginning at estrus (Day 0), ovaries were scanned for CL and for follicles -2 mm by transrectal ultrasonography daily for the first 25 days of pregnancy (PREG, n = 17 ewes) or until Day 8 of the second estrous

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... cle (CYC, n = 15 ewes). Jugular blood samples were collected at ultrasonography and on Days 30, 35, and 40 of gestation for RIA of progesterone and estradiol-17. Total follicles (17.6 ± 1.3), number of follicles 4 mm in diameter (11.3 + 0.6), and number of follicles that became the largest during the first 16 days of pregnancy or the first estrous cycle (4.9 0.3) did not differ between PREG and CYC ewes. Number of newly detected follicles each day and difference in mean diameter between the largest two follicles and all other follicles were similar between PREG and CYC ewes until luteal regression. At that time, difference in diameter between the largest two follicles and all other follicles increased in CYC ewes in parallel with estradiol-17{ in serum. Mean concentrations of progesterone in serum were higher in PREG than in CYC ewes (2.4 ± 0.1 vs. 2.0 ± 0.1 ng/ml; p < 0.05) before onset of luteal regression, but mean areas of CL did not differ. Transient declines in mean areas of CL and concentrations of progesterone occurred from Day 17 through Day 23 in PREG ewes; however, progesterone returned to midluteal phase values at 30 to 40 days. In conclusion, entry of follicles 2 mm in diameter into the pool of gonadotropindependent follicles occurred as a continuum. Luteal function was greater in PREG than in CYC ewes, but transient declines in both luteal size and function were observed during early pregnancy.