In the present study, we examined the clonic seizure immediately preceding head-weaving behaviour elicited by 8-OH-DPAT (40 mg/kg, ip) administration in mice. 8-OH-DPAT, known to be a central 5-HT1A receptor agonist, can induce a clonic seizure. Propranolol (1, 5, 10 mg/kg, ip) and methysergide (10, 20 mg/kg, ip) reduced 8-OH-DPAT (40 mg/kg, ip)-induced head-weaving behaviour and clonic seizure, while ketanserin (125, 250, 500 micrograms/kg, ip) was without effect. Trimethadione (500 mg/kg, sc) and phenobarbital (70 mg/kg, sc) completely inhibited clonic seizure and partially inhibited the head-weaving behaviour. Morphine (50 mg/kg, sc) completely inhibited both 8-OH-DPAT-induced head-weaving behaviour and clonic seizure. These effects of morphine are naloxone (20 mg/kg, sc)-reversible. These results suggest that the clonic seizure immediately preceding head-weaving behaviour elicited by 8-OH-DPAT is mediated mainly by serotonergic receptor 1A and also by additional factors.