We have recorded C-13 CP-MAS NMR spectra of Leu(5)- and Met(5)-enkephalins crystallized from a variety of solvent systems. It was found that the secondary structure of the enkephalins is well related to the manner of the peak-splitting in Tyr C, signals; the two distinct peaks were observed for the extended forms of Met(5)- and Leu(5)-enkephalins, whereas a broad single signal was observed for Leu(5)-enkephalin taking the P-bend form. We found that C-13 NMR spectra of freshly prepared Leu(5)-enkephalin mono- or trihydrate differ from those of samples contained in a rotor without airtight sealing. This was caused by irreversible conformational change owing to dehydration during magic angle spinning. In this connection, it was shown that conformational diversity of crystalline enkephalins was well characterized by C-13 NMR spectra. It is demonstrated that C-13 NMR spectra of the trihydrate recorded below -40 degrees C lead to an additional spectral change indicating the doubling of the peaks in Tyr C-xi and Phe C-delta peaks caused by the presence of conformational isomerism about the C-alpha-C-beta (defined by (chi 1) angle). The spectral profile of Tyr C-epsilon was well related to the presence or absence (or more precisely to the rate constant) of the flip-flop motion of the tyrosine side chain because the broad single peak is changed into a well-defined doublet peak when the sample was cooled down to -80 degrees C.