Laparotomies were performed on 17 lactating dairy cows, milked twice daily, 12 or 13 days after parturition to observe gross ovarian structures. Five cows were randomly given an IM injection of saline solution and 12 cows were given 100 micrograms of gonadotropin releasing hormone (GnRH). A 2nd laparotomy was performed 3 days later to observe changes in ovarian structures. In addition, the genital tract was palpated per rectum 5 to 8 days after the 2nd laparotomy, and blood samples for hormonal analysis were obtained prior to each laparotomy and palpation per rectum. Eight of 12 cows given GnRH and 2 of 5 cows given saline solution had developing corpora lutea with prominent ovulation papilla at the time of the 2nd laparotomy. Luteinized follicles were not observed. In GnRH-treated cows, 7 of 7 cows with follicles greater than or equal to 15 mm ovulated after GnRH treatment compared (P less than 0.01) with 1 of 5 cows with follicles less than or equal to 10 mm. Increase in plasma progesterone was not observed in GnRH-treated cows not ovulating. Thus, GnRH induced ovulation in postpartum dairy cows that had large follicles at the time of GnRH treatment.