Nociceptin/orphanin FQ (N/OFQ), a 17-amino-acid peptide, is an endogenous agonist whose receptor is similar in sequence to mu, delta and kappa opioid receptors. It has been reported that N/OFQ can block antinociceptive effects induced by opioid receptor agonists in the radiant heat tail-flick test and warm water tail-withdrawal test. The present study was designed to see the effect of N/OFQ on antinociception induced by opioid receptor agonists in the cold water tail-flick (CWT) test, which measures a different type of pain. In adult male Sprague–Dawley (S–D) rats given subcutaneous (s.c.) injections of saline or morphine (8 mg/kg), intracerebroventricular (i.c.v.) injection of N/OFQ (18 μg) 15 min later produced a significant reversal of morphine antinociception (P<0.01, ANOVA followed by Duncan's test), compared to the corresponding saline control group. Saline (t=+15 min, i.c.v.) had no effect on s.c. morphine antinociception (P>0.01), compared to the corresponding saline control group. When the kappa opioid receptor agonist spiradoline (80 mg/kg, s.c.) was used instead of morphine, similar results were observed. In another series of experiments, it was found that i.c.v. injection of N/OFQ (18 μg) reversed the antinociception induced by i.c.v. injection of the selective mu opioid agonist PL017 (2 μg), delta opioid agonist DPDPE (50 ng) and kappa opioid agonist dynorphin (21.5 μg), respectively. These results indicate that N/OFQ may be an endogenous anti-opioid peptide in the brain of rats in the CWT test.