Hydroxysteroid (17b)-dehydrogenase type 1 (HSD17B1) catalyzes the conversion of low active 17-ketosteroids, androstenedione (A-dione) and estrone (E1) to highly active 17-hydroxysteroids, testosterone (T) and E2, respectively. In this study, the importance of HSD17B1 in ovarian estrogen production was determined using Hsd17b1 knockout (HSD17B1KO) mice. In these mice, the ovarian HSD17B enzyme activity was markedly reduced, indicating a central role of HSD17B1 in ovarian physiology. The lack of Hsd17b activity resulted in increased ovarianE1:E2 and A-dione:T ratios, but we also observed reduced

progesterone concentration in HSD17B1KO ovaries. Accordingly with the altered steroid production, altered expression of Star, Cyp11a1, Lhcgr, Hsd17b7, and especially Cyp17a1 was observed. The ovaries of HSD17B1KO mice presented with all stages of folliculogenesis, while the corpus luteum structure was less defined and number reduced. Surprisingly, bundles of large granular cells of unknown origin appeared in the stroma of the KO ovaries. The HSD17B1KO mice presented with severe subfertility and failed to initiate pseudopregnancy. However, the HSD17B1KO females presented with normal estrous cycle definedby vaginal smears and normal pubertyappearance. This study indicates that HSD17B1 is a key enzyme in ovarian steroidogenesis and has a novel function in initiation and stabilization of pregnancy.