1. A cyclic dimeric nonapeptide neuropeptide Y (NPY) receptor antagonist, 1229U91, was synthesized by Fmoc chemistry and dimerised in solution. Its effects were assayed in mesenteric arteries from rats and mice, and in rat vas deferens. 2. Mesenteric arteries were cannulated and pressurised to 55 mmHg and the external diameters continuously measured. NPY, PYY, Leu31Pro34NPY and NPY(13-36) each caused concentration-related contractions with the order of potency PYY > or = Leu31Pro34NPY = NPY > NPY (13-36), consistent with the Y1 receptor subtype. 3. 1229U91 had no agonist activity in the arteries but caused a concentration-related rightward shift of NPY (mouse arteries) or Leu31Pro34NPY (rat) concentration-response curves. The antagonism was competitive with pKBS of 7.69 +/- 0.15 and 7.47 +/- 0.13 in the mouse and rat arteries, respectively. 4. Sympathetic nerves in the vas deferens were stimulated with a single electrical field pulse every 20 s and the twitch responses recorded. NPY, PYY, Leu31Pro34NPY and NPY(13-36) inhibited the twitches with the order of potency PYY > NPY > NPY(13-36) >> Leu31Pro34NPY, consistent with the Y2 receptor subtype. 5. 1229U91 inhibited the vas deferens twitch with a shallow concentration-response curve and a time-course of inhibition distinct from that of NPY. 1229U91 (30 microM) did not cause a rightward shift of the NPY concentration-response curve. 1229U91 is at least 5 orders of magnitude less potent in the vas deferens than in rat brain Y2 binding assays reported by others, suggesting that the brain and vas deferens Y2 receptors are different. 6. It is concluded that 1229U91 is a competitive antagonist of NPY Y1 vascular receptors and has additional properties that inhibit the electrically evoked twitch of the rat vas deferens.