Neuropeptide Y (NPY), a 36-residue peptide amide, has been shown by numerous studies to be a potent vasoconstrictor. In order to gain an appreciation of the structural requirements for this action, we have previously synthesized a number of fragments of NPY. It had been shown that sequential deletions from the N-terminus resulted in peptides with decreasing hypertensive activity. In the present study we present data supporting the unexpected finding of two fragments, NPY17-36 and NPT18-36 with substantial hypotensive action in vivo. This action was dose dependent (data not shown) and was also observed to a lesser extent with NPY19-36 but not NPY16-36 or NPY20-36. It was, however, slower in onset and of longer duration than the hypertensive action of NPY. These differing kinetics of action may suggest that NPY and NPY18-36 act through different mechanisms. Structural studies using circular dichroism were performed. While NPY was found to assume an ordered helical structure in both aqueous buffer and trifluoroethanol (TFE), 30% TFE in aqueous buffer was required to induce substantial helicity for NPY18-36. This structural investigation suggests that both NPY and NPY18-36 assume an ordered conformation upon reaching the lipid rich receptor environment.