Radioimmunoassay (RIA) was applied to identify selected metabolites obtained in in vitro incubations of ovarian and hepatopancreatic tissues from female Marsupenaeus japonicus in previtellogenic and vitellogenic maturation stages. Depending on the precursors used, progesterone, 17?-hydroxyprogesterone or testosterone, it was possible to observe the presence of several enzymatic activities through metabolites detected in incubations. The present study provides evidence that the ovary of the shrimp M. japonicus is capable of synthesising 17?-estradiol from progesterone, evidencing the presence of 17?-hydroxylase, C17-C20 lyase, 17?-hydroxysteroid dehydrogenase (17?-HSD) and aromatase. Enzymatic activities of aromatase, 3?-HSD and 17?-hydroxylase were detected in hepatopancreas, but not that of C17-C20 lyase. These results suggest that 17?-estradiol could be the vitellogenesis stimulating ovarian hormone (VSOH), and together with former in vivo and in vitro studies this leads us to propose that in fact there may be a physiological role for steroid hormones in the maturation cycle of crustaceans.