The cyclic dodecapeptide, cycloleonurinin, cyclo(-Gly-Pro-Thr-Gln-Tyr-Pro-Pro-Tyr-Tyr-Thr-Pro-Ala-), isolated from the fruits of Leonurus heterophyllus, showed potent immunosuppressive effect on human peripheral blood lymphocytes. The solution state conformation of cycloleonurinin was examined by high field NMR methods, distance geometry calculation and restrained energy minimization from NMR data. Calculation using 277 different initial structures led to a uniquely determined backbone conformation with a root mean square deviation value of 0.80 9. The backbone structure of cycloleonurinin consists of two β-turns, a β VI turn at Pro 6 -Pro 7 , and a β I turn at Pro 1 1 -Ala 1 2 . In addition to two transannular 4 → 1 backbone hydrogen bonds, which constructed two β-turns, two intramolecular hydrogen bonds between Tyr 9 -NH and Pro 7 -CO, and between Thr 1 0 -NH and Tyr 8 -CO, constructing γ-turns, and those between Thr 3 -NH and Tyr 8 -CO, and between Ala 1 2 -NH and Thr 1 0 -OH, were observed.