Solvent exchange rates of selected protons were measured by NMR saturation recovery method for yeast $tRNA^{Phe}$, at temperature from 25 to $40^{circ}C$, in the presence of 0.1 M NaCl and various low levels of added magnesium ion. The exchange rates in zero $Mg^{2+}$ concentration indicate early melting of acceptor stem, D stem, and tertiary structure. Addition of magnesium ion stabilizes the entire D stem more effectively than any other secondary or tertiary interactions.