We examined the effect of 20-hydroxyecdysone (20-HE), a neurosteroid found in insects that is involved in their developmental process, on both tonic convulsion and absence-like seizure in spontaneously epileptic rat (SER). When 20-HE was given orally to SER at 25-200 mg/kg, significant decreases of the tonic convulsion were observed with 100 and 200 mg/kg. Pretreatment of the animal with bicuculline (1 mg/kg, i.p.) antagonized the inhibitory effects of 20-HE. However, absence-like seizures were not affected by 20-HE. These findings indicate that 20-HE produces antiepileptic effects on tonic convulsion by acting on the modulatory site of GABA(A) receptors.