The enzyme 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) is essential for the biosynthesis of all steroid hormones. To date this laboratory has isolated and characterized five distinct 3 beta HSD cDNAs in the mouse (3 beta HSD I-V). The different isoforms fall into two functionally distinct groups. 3 beta HSD I and III function as dehydrogenase/isomerases and 3 beta HSD IV and V function as 3-ketosteroid reductases. Previously it was shown that the liver of the adult mouse expresses 3 beta HSD II, III and V, with 3 beta HSD III being the major isoform. This study examines the expression of the different forms of 3 beta HSD mRNAs and proteins in the livers of male and female mice during fetal and postnatal development. 3 beta HSD I, which in the adult mouse is expressed only in the gonads and adrenal glands, is the major isoform expressed in both male and female livers during fetal development until the first postnatal (pn) day after which time 3 beta HSD III becomes the major isoform. Expression of 3 beta HSD I mRNA and protein completely ceases after day 20 pn. The expression of 3 beta HSD V is first detected at day 40 pn and is observed only in the male. Very low expression of 3 beta HSD II mRNA is detected throughout development. Previous characterization of enzymatic activity of the expressed proteins showed that 3 beta HSD I exhibits lower Km values for the delta 5-3 beta-hydroxysteroids than 3 beta HSD III, indicating that 3 beta HSD I functions as a more efficient 3 beta-hydroxysteroid dehydrogenase/isomerase than 3 beta HSD III. The results of this study suggest that the liver may play an important role in the biosynthesis of steroid hormones during murine fetal development.