Specific inhibition of type 1 3beta-HSD is of particular interest since it will allow us to control the formation of androgens and estrogens in peripheral target tissues without affecting type 2 3beta-HSD, which is responsible for the biosynthesis of glucocorticoids and mineralocorticoids in the adrenals. The high homology between types 1 and 2 3beta-HSD is a major difficulty in the development of specific inhibitors through classical chemical synthesis. In this report, we describe the use of small interference RNA (siRNA) to specifically inhibit human type 1 3beta-HSD. We have constructed three DNA vector-based RNAi vectors that allow us to produce three RNA duplexes of 21 nucleotides targeting three different coding regions of human type 1 3beta-HSD mRNA. The resulting constructs were co-transfected into HEK-293 cells with a vector expressing type 1 3beta-HSD. The results indicate that while the two duplexes that target sequences in the 5'-region do not have a strong inhibitory effect, the duplex that targets the 3'-region efficiently inhibits 3beta-HSD activity. Up to 98% inhibition has been observed. To our knowledge, this is the first report showing successful inhibition of steroidogenic enzymes using siRNA technology.