Several hormonal treatments to synchronize ovulation for cattle breeding require considerable funds and labor. To increase the economic efficiency of medicine and to reduce labor in the synchronization of the ovulation of cows, a silicon rubber coating stainless steel device impregnated with 1.55 g of progesterone, i.e., a progesterone releasing intra-vaginal device (PRID−E), was inserted into the vagina of Holstein cows for 20 days from Day 2 (D2 group, day 0 is the day of ovulation) or Day 15 (D15 group). There are 3 Holstein cows in each group. Changes in the ovaries, blood hormone levels and ovulation were examined to learn about the potential applicability of the treatment in the synchronization of estrus and ovulation. Ovulation occurred 3‐4 days after withdrawal of the device in all the cows of both groups. The number of follicular waves that appeared during the PRID−E treated ovarian cycle was 2‐3 in the D2 group and 3‐4 in the D15 group. In both groups, the ovulated follicles dominant period was 7 days in 2 of the cows in the group and the ovulated follicles dominant period in the remaining cow in either group was 13 or 14 days. The maximum diameter was 14.1‐17.9 mm in 5 of the cows in the two groups, but the remaining cow in the D2 group was 20.5 mm. Signs of estrus appeared and became obvious the day after the withdrawal of the device. As a result, treatment using a PRID−E for 20 days from Day 2 or Day 15 would synchronize ovulation to 3‐4 days after the withdrawal of the device.