To further elucidate the minimum bioactive conformation of neuropeptide Y (NPY), a series of truncated and conformationally constrained analogues has been prepared. The synthesis and purification of these peptides was achieved using routine laboratory strategies and techniques. Parent molecules consisted of the native NPY N-terminal 1-4 and C-terminal 25-36 segments, having the residue 5-24 core replaced by either a single flexible omega-aminoalkanoic acid, or a more rigid Pro-Gly or Pro-DAla sequence which was expected to constrain a putative turn, and allow the N- and C-termini to align. Cross-linking between residues 2 and 27 through lactamization using side-chain length and chirality suggested by computer simulations, resulted in cyclo-(2/27)-des-AA7-24[Glu2,Gly6,DDpr27]NPY that exhibited very high affinity (Ki = 0.3 versus 0.3 nM for NPY) for the Y2 receptor using SK-N-BE2 human neuroblastoma cells, yet very low affinity for the Y1 receptor using SK-N-MC human neuroblastoma cells (Ki = 130 versus 2.0 nM for NPY). The added constraint resulting from bridging in this analogue as well as in others suggested that the combination of the deletion of residues 5-24 and the introduction of an internal ring produced exclusive selectivity for the Y2 receptor with little or no loss of affinity. The tolerance of structural recognition was further demonstrated as a second ring was introduced which was expected to constrain the amphiphilic alpha-helix, resulting in the full Y2 agonist dicyclo (2/27,28/32)-des-AA7-24 [Glu2,32,DAla6,DDpr27,Lys28]NPY. Improvement of Y1 binding activity was achieved only by including more residues (des-AA10-17) in the central PP-fold region, while allowing limited flexibility of the termini. Although the length of the bridge seemed to have little effect on binding potency, changes in the location of and chirality at the bridgehead resulted in analogues with different binding affinities. Combination of optimum structural modifications resulted in cyclo-(7/21)-des-AA10-18[Cys7,21]NPY, an analogue shortened by 25% but retaining comparable binding properties to that of native NPY at Y1 and Y2 receptor types (Ki = 5.1 and 1.3 nM, respectively).