D-7400 Tubingen, *Abteilung Pharrnakologie, and 3Abteilung Pharmachemie, Boehringer lngelheim KG, D-6507 Ingelheirn, Germany SYNOPSIS C-terminal analogues of neuropeptide Y ( N P Y ) of small molecular size have been synthesized. The influence of chain length, single or multiple amino acid substitution, and segment substitutions on receptor binding, pre-and postsynaptic biological activity, and conformational properties have been investigated. Receptor binding and in vivo assays revealed biological activity for NPY Ac-25-36 that increased with increasing a-helicity. In attempts to stabilize the a-helical content, three independent types of modified NPY Ac-25-36 analogues were synthesized. Strong agonistic activities could be detected in a series of discontinuous analogues, which are constructs of N-terminal parts linked via different spacer molecules to C-terminal segments. One of the most active molecules was NPY 1-4-Aca-25-36 ( Aca, t-aminocaproic acid). For the first time conformational properties of a series of small NPY analogues have been investigated by CD, and correlated with biological activity and receptor binding. A C-terminal dodecapeptide segment of NPY with an amount of 50% substitution to the native C-terminal sequence of NPY was found to exhibit significant receptor binding.