The conformational role of Thr was investigated by 13C-n.m.r. and CD methods using a following series of tetrapeptides: Thr-Ala-Ala-Ala, Ala-Thr-Ala-Ala, Ala-Ala-Thr-Ala and Ala-Ala-Ala-Thr. It was found that introduction of Thr in every position of the tetraalanine peptide chain distinctly influences conformational equilibria of the peptides. An increase of β-turn forms in conformational equilibria is induced by ionization of the terminal carboxyl group, independent of threonine position in the peptide chain. Threonine in position 1 or 3 of the peptide chain seems to have some importance for β-turn formation in acid solution.