The N-terminal 1-34 fragment of parathyroid hormone (PTH) is fully active in vitro and in vivo and it can reproduce all biological responses characteristic of the native intact PTH. Recently, analogues of PTH(1-11) fragments with helicity-enhancing substitutions have been demonstrated to yield potent analogues of PTH(1-34). The work describes the synthesis, biological activity and structure of analogues of the best modified PTH sequence H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH2 (I). In particular, the effect of the Ala/Aib substitution at positions 1 and 3 as well as of the replacement of Nle in position 8 with D-Nle, L-(αMe)-Nle and D-(αMe)-Nle was studied. The resulting peptides were characterized structurally by CD spectroscopy, solution NMR and MD, and in vitro for activity with respect to the cognate receptor, parathyroid hormone receptor.