NMR spectroscopy of the E-domain fragment of Escherichia coli 5S rRNA indicates that this molecule exists in solution as either a stem-loop or as a duplex with two U-U base pairs in the bulge region. At temperatures below 27 degrees C, interconversion between the monomeric and dimeric forms in solution occurs on a time scale of weeks and allows the preparation of samples on which NMR structure determinations can be carried out on predominantly monomeric or dimeric species. The NMR results obtained provide comparison data for the distinction between A- and B-form E.coli 5S rRNA and for the possible kinetics of conversion between these forms. NMR evidence is presented that the duplex form also exists in crystals and suggestions are made for means to obtain stem-loop conformations of E-domain and other small RNA stem-loop sequences in crystals.