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The role of peripheral and spinal 5-HT₃ receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia in both paws. In experiments where the test drug was anticipated to augment or antagonize the response, 0.5 or 1% formalin, respectively, was used for injection. Peripheral ipsilateral, but not contralateral, pre-treatment (−10min) with serotonin (5-HT, 10–100nmol/paw) and the selective 5-HT₃ receptor agonist 1-(m-chlorophenyl)-biguanide (m-CPBG, 10–300nmol/paw) increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. Moreover, spinal pre-treatment with m-CPBG (10–300nmol/rat) increased 0.5% formalin-induced secondary hyperalgesia but not allodynia in both paws. Accordingly, peripheral …