The 95 % ethanol extract of Gelsemium sempervirens showed inhibitory activity against human DNA topoisomerase I (Topo I). Phytochemical investigations of this active extract resulted in the isolation and identification of three new steroids ( 1 - 3), together with eight known compounds 12 beta-hydroxy-5 alpha-pregn-16-ene-3,20-dione ( 4), gelsemine ( 5), sempervirine ( 6), scopoletin ( 7), 7- O- beta- D-glucopyranosylscopoletin ( 8), 7- O- beta- D-apiofuranosyl-(1-->6)- beta- D-glucopyranosylscopoletin ( 9), uvaol ( 10), and 2-(4-hydroxyphenyl)ethyl heptadecanoate ( 11). The structures of the new steroids were determined by extensive NMR and HR-ESI-MS analyses as 21-hydroxy-5 alpha-pregn-16-ene-3,20-dione ( 1), 3-oxoandrosta-16-ene-17-carboxylic acid ( 2), and 3-oxoandrosta-4,16-diene-17-carboxylic acid ( 3). This study suggests that sempervirine ( 6) intercalates to DNA and also inhibits Topo I through modulating the enzyme activity with an IC (50) of 54.5 +/- 15.9 muM.