We report results of numerical analyses on solvation structure and conformational stability of a dipeptide and Met-enkephalin in the extended simple point charge (SPC/E) model water. The reference interaction site model (RISM) theory is fully solved using our robust, highly efficient algorithm, It is shown that water structure near the peptides and the hydration free energy are greatly dependent on the peptide conformations. Stability of Met-enkephalin is examined in terms of the total energy defined as the sum of the conformational energy and the hydration free energy of the peptide. We test several different conformations including that with the minimum energy in gas phase, which tal;es rather compact form due to an intramolecular hydrogen bond. It is shown that a fully extended conformation has the highest stability in water. Our results are in qualitative accord with the recent nuclear magnetic resonance (NMR) experiments which suggest fully extended conformations with large fluctuations for the solution structure of the peptide. A conformation which is similar to that obtained from the NMR experiments in miceller solutions, is much less stable when it is put in water. Thus, the peptide conformations are greatly sensitive to microscopic solvent environment, and any native treatment of the solvent such as the continuum model will end in failure.