The newly introduced antiepileptic drug, lamotrigine, has been reported to have a mechanism of action similar to that of phenytoin. Because phenytoin is a standard clinical treatment for convulsive status epilepticus, we compared the efficacy of lamotrigine to that of phenytoin in a model of secondarily generalized convulsive status epilepticus in rats that responds to drug concentrations similar to those that have been reported to be clinically useful for this purpose. Status epilepticus was induced in rats with actively epileptogenic cortical cobalt lesions by administration of homocysteine thiolactone. While phenytoin-controlled generalized tonic clonic seizures in this model with a median effective dose of 100.5 mg/kg (16.0 micrograms/ml in serum), lamotrigine was ineffective at doses ranging from 10 to 100 mg/kg, with serum drug concentrations (2.5-43.5 micrograms/ml) within or above the reported 'therapeutic' concentration for LTG treatment of chronic epilepsy. Lamotrigine also failed to prevent the onset of generalized tonic clonic seizures when given prior to homocysteine, while phenytoin was effective in this test. Studies of lamotrigine kinetics in serum and brain revealed that the drug was well-absorbed following i.p. injection and that it entered brain rapidly enough to have exerted an anti-status effect in these experiments. These results suggest that lamotrigine and phenytoin have differences in their mechanisms of anticonvulsant action, leading to very different abilities to control status epilepticus.