This study was designed to specify the testosterone (T), estradiol-17β (E2) and 17,20
β-P) variations during the pre-ovulatory period in female Eurasian perch, Perca fluviatilis, and also examine the ovarian synthesis of 17,20β-P and 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S). Five females (100–300 g) were collected from a concrete pond from the Lindre Center (Moselle, France) between March and April 1999, three times a week, and sex steroids measured. Two females were also collected from the Lindre pond on three dates (18 February, 14 and 23 March) for an in vitro study. Significant plasma 17,20β-P peaks were measured for the first time in this species reaching 3.5 ng ml–1. However, the synthesis and release of 17,20β-P in blood seems to not be synchronized with the final maturation and ovulation stages. The in vitro study showed that ovaries at the pre-ovulatory stage produce some 17,20β-P but no 20β-S from a tritiated precursor (17-P-3H). 17,20β-P is produced after 30 min and is still present in the medium after 2 h of incubation, but is no longer detectable after 4 h. Some other non-identified compounds were synthesized at this stage suggesting that 17,20β-P was quickly metabolized. Moreover, the plasma level increases, observed in spawned females suggest that this steroid might be involved in the oviposition process. These results suggest a role for 17,20β-P in the final stages of oocyte development.