Postpartum resumption of ovarian activity in 40 Holstein cows was monitored by ultrasonography twice weekly until artificial insemination. The accuracy of ultrasonography for assessments of ovarian structures was examined by comparing results of in vivo ultrasonography with macroscopic findings of the same ovaries after slaughter. Correlation coefficients were 0.71 and 0.85 for number of follicles 10-14 mm and > or = 15 mm, and 0.99 for diameters of the largest follicle. Follicular profiles prior to first ovulation were characterized by single dominant follicle (DF > or = 10 mm) in 25 cows, two in 10, three in 4, and four in one, respectively. However, after first ovulation, two waves of DF prevailed. The total number of DF (7.2) or time of ovulation (3.6) before conception was positively correlated with postpartum intervals to conception (74.0 days). Profiles of the volume of corpus luteum estimated by ultrasonography paralleled with the variations of plasma progesterone levels. The volume of corpus luteum and the peak progesterone level were smaller after the first ovulation as compared with after the second or third ovulations. In the ultrasound images of uterus, two elliptical lines indicated cross section of endometrium and stratum vascularis. Uterine involution assessed by reaching the nadir of endometrium was completed by 41.5 days postpartum. Results indicated that the number of DF before the first ovulation and the volume of corpus luteum after the ovulation were smaller compared with those of the second and third ovulations.