We explore THz optical activity of ensembles of dense randomly oriented metallic helices by studying their time-domain response to THz electromagnetic pulse excitation. The interaction of the electromagnetic wave with this artificial chiral material mimics that interaction with highly concentrated optically active liquids. By dynamically accessing the THz electric field transmitted through the helical chiral media, optical activity signatures are correlated with the arrival time and polarization state of the detected THz electric field radiation. Our experiment show two distinct phases for optical rotation: one which is associated with scattering and the other is associated with phase accumulation.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.