Pregnancy was maintained in Long-Evans rats hypophysectomized on day 1 and injected with ovine pituitary gonadotrophins from day 1 through day 11; and also in rats hypophysectomized on day 6 and injected from day 6 through day 11. For successful maintenance until term, no further injections were necessary after day 11. The combination of hormones considered close to optimal was; mammotrophin 2.5 mg; follicle-stimulating hormone 25 μg; and interstitial cell-stimulating hormone 5 μg injected daily, subcutaneously. Histological examination of ovaries after this regimen (on day 12) revealed interstitial and luteal tissue that was considered normal for this stage; and follicles that were in some cases larger than expected and occasionally with theca luteinization. The ovaries of the rats allowed to go to term and untreated after day 11 showed follicular and interstitial tissue that had regressed and well-stimulated corpora lutea. These findings showed no evidence of placental FSH or ICSH-like hormones; but confirmed earlier reports of a rat placental luteotrophic hormone.