Following rapid IV injection (0.1 mg per kilogram) lidocaine HCl concentrations were measured in the blood and brain of paralyzed, ventilated rats during bicuculline-induced status epilepticus and in identically prepared controls. The concentration of lidocaine in blood and brain was consistently higher in convulsing than in nonconvulsing rats. At 1 minute, increased brain lidocaine reflected elevated blood concentrations; increased brain and blood partitioning after 1 minute is responsible for subsequent increases in brain lidocaine uptake. The therapeutic index of lidocaine is low; the concentration of lidocaine is increased in the convulsing brain. Our data suggest that conventional lidocaine doses may perpetuate rather than control refractory convulsions.