An important effect on the ovaries of hypophysectomized rats after high doses of pregnant mare serum gonadotrophin (PMSG) is follicular development. Even though this action is combined with distinct interstitial cell stimulation (Cole, Pencharz & Goss, 1940), PMSG has been used as a follicle-stimulating hormone (FSH) preparation in gynaecology and andrology for many years. Dörner & Hohlweg (1961), however, failed to demonstrate synergism between PMSG and human chorionic gonadotrophin (HCG) on the ovary of rats, comparable to that obtainable with hypophysial FSH + HCG, but they found a marked synergism between PMSG and FSH. These findings suggested a predominantly interstitial cell-stimulating activity of PMSG. Further studies on the quantitative characterization of the ratio of follicle-stimulating to interstitial cell-stimulating activity of PMSG (Gestyl, Organon; Nobigonan-S, Nobilis) as compared with human menopausal gonadotrophin (HMG) (Humegon, Organon), hypophysial porcine FSH (Folistiman, Arzneimittelwerk Dresden) and human hypophysial FSH (Anthrogon, Arzneimittelwerk Dresden) were done. Follicle-stimulating activity