The carbon-13 (13C) nuclear magnetic resonance (NMR) behavior of the hyperactive analogue of luteinizing-hormone-releasing hormone (LH-RH), des-Gly-NH2(10)-[D-Leu6]LH-RH ethylamide, has been studied. The spectra are compared with those of the hypoactive des-Gly-NH2(10)-[L-Leu6]LH-RH ethylamide. Spin-lattice relaxation times (T1) of 13C at natural abundance have been obtained for both peptides at 25 and 68 MHz in aqueous solution and used as monitors of flexibility at each carbon atom in the peptide backbone and side chains. The [D-Leu6]-and [L-Leu6]-sub-stituted analogues show equal degrees of freedom within the peptide backbone and side chains. Substitution of the glycine amide moiety in position 10 by an ethylamide group causes an increase in flexibility of the backbone at that position. The increase in flexibility is not transmitted to the adjacent prolyl residue. The C-terminal peptide link at position 10 is proposed to restrict the flexibility of the prolyl residue in position 9, thus maintaining the motional characteristics of the LH-RH peptide backbone in the des-Gly-NH2(10)-[Leu6]LH-RH ethylamide analogues. There were no significant differences in the 13C chemical shifts or T1 values between the hypoactive and hyperactive analogues in aqueous solution which would explain the differences in biological activity. Interaction between receptor and hormone may thus be required to induce, or select, the biologically active conformation of the hormone.