Abstract. Transrectal ovarian ultrasonography was performed to examine follicular and luteal dynamics during the estrous cycle in four adult female Shiba goats. Detectable follicles and the corpora lutea in both ovaries were monitored daily for 44 days using a B-mode scanner with a 7.5 MHz transrectal transducer. Blood samples were collected by jugular venipuncture at the time of ultrasonography and analyzed for estradiol-17β and progesterone by radioimmunoassay to monitor ovarian activity. Three to six follicles, whose diameters ranged from 1 to 7 mm, were observed throughout the estrous cycle; a few follicles grew to more than 5 mm in diameter and most of them atrophied during the luteal phase. Ovulatory follicles appeared from 5 days before ovulation coinciding with the start of the regression of corpora lutea, and then grew rapidly into 5.5 ± l.0 mm (mean ± SD) in diameter in parallel with the increase in the plasma concentration of estradiol-17β. The corpora lutea, 6.1 ± 1.3 mm in major axis, were detected from 3.0 ± 1.6 days after ovulation and stopped growing on 6 to 8 days after ovulation, with a major axis of 8.1 ± 2.2 mm. The mean length of the major axis of fully developed corpora lutea remained constant until 14 days after ovulation and then decreased rapidly to 5.2 ± 1.7 mm on the day of the next ovulation. In the early and functional luteal phase, there was a positive correlation between mean length of the total major axis of corpora lutea and mean plasma concentration of progesterone, whereas in the late luteal phase, the mean length of the total major axis of corpora lutea decreased slower than the progesterone levels. The present study demonstrated the ovarian follicular and luteal dynamics throughout the estrous cycle, indicating that ultrasonography is a reliable means for monitoring ovarian activity in Shiba goats.

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