The possibility of selectively reducing the number of beta-helical structures theoretically possible for a D,L-alternating peptide by using a N-methyl group as conformational constraint is considered. Some 1H-nmr data regarding Boc(L-Nle-D-Nle)3-L-Nle-D-MeNle-L-Nle-D-Nle-L-Nle-OMe (I), its formyl analogue (II), and the pentadecapeptide Boc(D-Leu-L-Leu)5-D-MeLeu-(L-Leu-D-Leu)2-OMe (III) are presented. It is shown that these alternating stereocooligopeptides with a N-methyl group in the (n - 3) (I and II) or (n - 4) position (III) differ drastically in their behavior from the corresponding nonmethylated compounds. In chloroform, I and II form predominantly -- beta 7.2-helices and III forms almost exclusively -- beta 5.6 or -- beta 7.2-helices. The helices are in every case those having the maximum possible number of interchain H bonds.