Orphanin FQ（OFQ） or nociceptin is a novel neuropeptide consisting of 17 amino acids. This peptide has a primary structure reminiscent of that of opioid peptide but exhibits an opposite effect to make animals hyperreactive. The effect of this new peptide on cardiovascular function are not completely known. The present study was conducted to investigate the effect of intravenous bolus injection of orphanin FQ on mean arterial blood presure （MABP） in conscious stroke-prone spontaneously hypertensive rats (SHR

sp

). Adult male SHR

sp

and Wistar normotensive rats （250～300 g body weight

2. 5～3 months old） were used in this study. The MABP was measured in the conscious state by a tail-cuff method. In SHR

sp

model

intravenous bolus injection of orphanin FQ or Tyr

1

-orphanin FQ （0. 5 mg/kg） induced a prolonged and marked reduc- tion in MABP. The maximum changes in MABP were -30. 2±4. 2 mmHg by orphanin FQ and -28. 2± 4. 7 mmHg by Tyr

1

-orphanin FQ at 10 min after administration

and this effect lasted over 30 min. The Phe

1

→Tyr substitution in orphanin FQ was found to retain almost fully hypotensive activity. Pretreatment of SHR

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with naloxone-HCI（60 μg/kg）

5 min before the injection of orphanin FQ

did not block the hy- potensive effect of orphanin FQ. Therefore

opioid receptors could not account for the hypotensive effect of orphanin FQ in SHR

sp

. In Wistar rats

intravenous bolus injection of the same dose of orphanin FQ did not cause a change in MABP. These observations suggest that orphanin FQ is a novel hypotensive peptide and may have some role in the regulation of blood pressure in SHR

sp

rather than in normotensive rats. The ex-act underlying mechanisms are waiting to be clarified.