The ability of prolactin to prevent luteinization of follicles in response to exogenous luteinizing hormone (LH), and PMSinduced ovulation and luteinization, was investigated in prepubertal rats whose first estrus and ovulation were synchronized by the subcutaneous (sc) administration of 4 IU pregnant mare serum gonadotropin (PMS). Pseudopregnancy was induced by the sc administration of prolactin (100 μg NIH-P-B2 administered twice daily) beginning on the day of ovulation (day 0) and maintained through day 2. The sc administration of LH (50 μg NIH-LH-B6) in sesame oil containing 5% beeswax once daily on days 3, 4, 5 and 6 resulted in the presence of almost twice as many corpora lutea at autopsy on day 10 as were present following administration of the vehicle alone. This response to LH probably involved luteinization of the follicles without ovulation. Concomitant administration of prolactin (100 μg NIH-P-B2 administered twice daily) or 8 mg of progesterone (4 mg twice daily), but not 2 mg of progesterone (1 mg twice daily), abolished this response to LH. A second administration of PMS on day 3 failed to induce ovulation when prolactin was continued until autopsy. Withholding prolactin on days 3 through 6 enabled PMS to induce new ovulations. Administration of high levels of progesterone (8 mg daily) on days 3 through 6 also abolished PMS-induced ovulation. More moderate levels of progesterone (2 mg daily) failed to prevent a PMS response in all the animals. Prolactin did not prevent PMS-induced ovulation in prepubertal rats where no corpora lutea were present, or in “synchronized” rats whose first crop of corpora lutea were destroyed by electrocautery. These results suggest that the anti-ovulatory action of prolactin is secondary to maintenance of progesterone secretion from the corpora lutea. (Endocrinology89: 584, 1971)