Porcine neuropeptide Y (NPY), NPY fragments, and analogues with D-Xaan, Ala9, D-Ala9, and Met17 substitutions or modifications to the C- or N-termini were synthesized. The synthesis and purification of these peptides was achieved by using routine laboratory strategies and techniques. The ability of these peptides to alter mean arterial pressure (MAP) and heart rate (HR) in conscious rats was monitored for 15 min following intraarterial administration. Potencies and efficacies of these peptides relative to NPY were determined by comparison of dose-response curves. Administration of 40 micrograms/kg NPY resulted in a rapid, though short-lived, rise in mean arterial pressure from a basal value of 107.0 +/- 2.6 to 157 +/- 5.5 mmHg (means +/- sem, n = 13). The ED50 (+/- SE) for this response was 3.04 +/- 0.88 micrograms/kg. Peptide YY (PYY) elicited a response that was similar in magnitude but with an ED50 (+/- SE, n = 3) of 0.76 +/- 0.24 micrograms/kg while porcine pancreatic polypeptide (pPP) was inactive when tested at 40 micrograms/kg (n = 4). Relative potencies for [Ac-Tyr1]NPY, [Ac-D-Tyr1]NPY, [des-amino-Tyr1] NPY, and [Me-Tyr1]NPY ranged from 1.1 to 2.2. Potencies relative to NPY for D-substitutions at positions 2-6 and 8-13 inclusive ranged from 0.1 to 1.0. Analogues with D-substitutions at positions 1-3 exhibited an extended duration of action. Analogues with D-substitutions at positions 33-35 inclusive were inactive at 40 micrograms/kg, and [D-Tyr36]NPY was 10-fold less potent than NPY, suggesting that the integrity of the C-terminal region is critical to the overall biological action of NPY. This conclusion is supported by studies with C- and N-terminal deletion peptides. NPY2-36 showed full intrinsic activity at 40 micrograms/kg and retains 40% of the hypertensive potency of NPY. There was a sequential decrease in efficacy upon further N-terminal deletion. In contrast to the finding with NPY2-36, modification of the C-terminus either from the native carboxamide to the free carboxylic acid or by deletion of the C-terminal residue resulted in analogues which were inactive at 40 micrograms/kg. These data indicate that an essentially full-length, C-terminally amidated NPY structure is required for the hypertensive activity observed in conscious rats upon intraarterial administration of NPY and NPY analogues.