The high-resolution structures of two separate RNA molecules were determined using heteronuclear nuclear magnetic resonance (NMR) spectroscopy. Both RNA molecules, in their respective systems, perform critical functions of regulating gene expression. To explore the dynamic properties of one molecule, an extensive investigation into its fast (<5ns) intra-molecular motions was carried out. The dynamics investigation was performed using a novel application of heteronuclear relaxation measurements from several base and ribose sites within the molecule. The results of this work were then interpreted using various forms of the Lipari and Szabo motional model. The significance of this research is two fold: (1) it provides detailed structures of two RNA molecules and a basis for their roles in protein recognition and binding and (2) augments the structural characterization with direct measurements of fast internal motions using a novel application of heteronuclear NMR relaxation analysis.