The anticonvulsant effects of levetiracetam were assessed in two genetic rat models. In the audiogenic-seizure prone rat, levetiracetam, 5.4 to 96 mg/kg i.p. dose-dependently inhibited both wild running and tonic-clonic convulsions. In the GAERS model of petit mal epilepsy, levetiracetam markedly suppressed spontaneous spike-and-wave discharge (SWD) but left the underlying EEG trace normal. The effects were already marked at 5.4 mg/kg and did not increase significantly up to 170 mg/kg although more animals were completely protected. Levetiracetam produced no observable effects on behaviour apart from slight reversible sedation at 170 mg/kg. In contrast, piracetam, a structural analogue of levetiracetam, significantly and consistently suppressed SWD in GAERS rats only at the high dose of 1000 mg/kg with some slight effects at lower doses. The effect of piracetam appeared to be due to increased sleeping rather than to a direct antiepileptic effect. The results with levetiracetam argue for a clinical application in both petit mal, absence epilepsy and in treating generalised tonic-clonic and partial seizures.