In the presence of indomethacin, an inhibitor of prostaglandin (PG) synthesis, the gonadotropin surge induces abnormal follicle rupture at the basolateral follicle sides, thus preventing effective ovulation in rats. This study was undertaken to analyze whether exogenous prostaglandin administration can overcome the antiovulatory action of indomethacin. Cycling rats were treated with vehicle (olive oil) or indomethacin (1 mg/rat) on the morning of proestrus. Rats treated with indomethacin were injected with different doses (50, 250, or 500 μg/rat) of PGE₁, PGE₂, PGF_2α, or vehicle (saline) at 1900 h in proestrus. The ovulatory response was analyzed on the morning of estrus by evaluating follicle rupture and the location of the oocytes in serially sectioned ovaries. The number of oocytes in the oviducts was also counted in rats treated with the highest prostaglandin doses. In indomethacin-treated rats, most newly formed corpora lutea showed abnormal follicle rupture at the basolateral sides. In addition, invasion of the ovarian stroma and blood and lymphatic vessels by granulosa cells and follicular fluid was observed. Prostaglandins of the E series, and especially PGE₁, inhibited abnormal follicle rupture and restored ovulation, although the number of oocytes in the oviducts were significantly decreased. PGF_2α was only partially effective in inhibiting abnormal follicle rupture and restoring ovulation. These data suggest that prostaglandins of the E series, and particularly PGE₁, play a crucial role in ovulation by determining the targeting of follicle rupture at the apex, thus allowing release of oocytes to the periovarian space.