F\l=O"\LDES 19-Hydroxylation of the steroid molecule is an obligatory step in the conversion of androgens to oestrogens. It has been shown (F\l=o"\ldes,Koref, Feh\l=e'\r & Steczek, 1964) that the urinary excretion of individual oestrogens decreases after the administration of 2-methyl-1,2-bis-(3-pyridyl)-1-propanone (SU4885).The aim of the present communication is to show that SU 4885 inhibits the transformation of androstenedione to oestrogens in the rat ovary in vitro. The following trivial names are used: androstenedione = androst-4-ene-3,17-dione; oestradiol = oestra-1,3,5(10)-triene-3,17\g=b\-diol; oestrone = 3-hydroxy-oestra-1,3,5(10)-trien-17\x=req-\ one.Female rats of the Wistar strain, weighing 180\p=n-\250 g., received two injections of 50 units chorionic gonadotrophin (HCG, Choriogonin, Richter, Budapest) 24 and 16 hr. before the ovaries were removed. For each assay, 18 animals were used, six of which also received s.c. 10 mg. SU 4885 (Metopirone, Ciba, Basel) 40, 24 and 16 hr. before death. The ovaries were removed, halved and distributed (12 halved ovaries, 250-350 mg./vessel) between six vessels each containing 5\m=.\0ml. Krebs\p=m-\Ringer phosphate solution, pH 7\m=.\4.Tissue from the SU4885-treated animals was placed in vessels 3 and 4. Androstenedione, 25 pig. in 0-1 ml. propylene glycol, was added to vessels 2, 4 and 6 and propylene glycol only to the remaining three, which then served as controls. SU4885 (20 pig.) was also added to vessels 5 and 6. Incubations lasted for 3 hr. at 37°in 95 % 02:5 % C02.After incubation, the contents were homogenized and extracted three times with 15 ml. peroxide-free ether. The combined extracts were washed with 5 ml. water, dried over anhydrous'Na2S04 and the ether evaporated. Thin-layer chromatography was used for purification and separation of the oestrogens. The extracts and 0-25, 0-5 and 1-0 pig. of oestradiol and oestrone (for a standard curve) were applied to 1 cm. strips of Kieselgel G chromatoplate (layer thickness 250 pi), equilibrated for 20 min., developed for 2 hr. in freshly distilled chloroform for purification and for separation in the system benzene : absolute ethanol (95:5). The zones containing oestradiol and oestrone were located from a reference strip, scraped off, and Ittrich's (1958) colour reaction performed in the presence of the adsorbent. Oestradiol and oestrone were determined by means of calibration curves.