This Letter contributes to the development of the first-principles prediction methods for peptide and protein conformations in aqueous solution. We report the results of the first attempt to combine the multicanonical algorithm for extensive sampling of the phase space and the reference interaction site model (RISM) theory for incorporating solvent effects. Met-enkephalin in aqueous solution is chosen as an example system. Averages of the energy functions, end-to-end distance, and dihedral-angle distributions are calculated as functions of temperature.