AbstractThe13C chemical shifts and spin‐lattice relaxation times are reported forcyclo(L‐Pro‐L‐Leu) andcyclo(L‐Pro‐D‐Leu). The chemical shifts of theDandLleucyl residues in the cyclic peptides differ from each other by 1.8 and 3.6 parts per million for the α and β carbons, respectively. The α‐carbons of the prolyl residues differ by 1.0 ppm as a consequence of proximity to aDor anLleucyl residue. The13C spin‐lattic relaxation time(T1) of the prolyl residues, but not the leucyl residues, in both compounds are indicative of difference in conformational equilibria within the pyrrolidine ring in theL‐Lisomer as compared to theL‐Disomer. Anisotropic overall molecular reorientation is not responsible for the differences observed in theT1values. The differences inT1values and chemical shifts betweencyclo(L‐Pro‐L‐Leu) andcyclo(L‐Pro‐D‐Leu) appear to result from a difference in conformatio