Estradiol (E2) plays a crucial role in all reproduction processes. In the placenta, it is well recognized that E2 is synthesized from fetal dehydroepiandrosterone sulfate (DHEAS). However, there is some controversy about the biosynthetic pathway involved, some authors suggest that E2 is produced by aromatization of testosterone (T), while others suggest that E2 is produced by the conversion of estrone (E1) into E2 by type 1 17beta-HSD, subsequent to the aromatization of 4-androstenedione (4-dione) into E1. In the present report, using the precursor [(14)C]DHEA, inhibitors of steroidogenic enzymes (chemical inhibitors and siRNA) and a choriocarcinoma (JEG-3) cell line that expresses all the enzymes necessary to transform DHEA into E2, we could determine the sequential steps and the specific steroidogenic enzymes involved in the transformation of DHEA into E2. Quantification of mRNA expression levels using real-time PCR, strongly suggests that type 1 3beta-hydroxysteroid dehydrogenase (3beta-HSD1), aromatase and type 1 17beta-HSD (17beta-HSD1) that are highly expressed in JEG-3 cells are the enzymes responsible for the transformation of DHEA into E2. Analysis of the intermediates produced in the absence and presence of 3beta-HSD, aromatase and 17beta-HSD1 inhibitors permits to determine the following sequential steps: DHEA is transformed into 4-dione by 3beta-HSD1, then 4-dione is aromatized into E1 by aromatase and E1 is finally transformed into E2 by 17beta-HSD1. Our data are clearly in favor of the pathway in which the step of aromatization precedes the step of reduction by 17beta-HSD.