A series of newly synthesized N-phenyl-substituted derivatives of succinimide were screened for anticonvulsant activity. Addition of a sulfonamide group in the p-position was of great consequence for the anticonvulsant effect. Substitution of a halogen in the m- or o-position improved activity against electroshock induced seizures. Pentylenetetrazole convulsions could only be prevented by few of these substances in smaller than 200 mg/kg oral doses. Activity could be further enhanced by adding more aliphatic or aromatic groups to the succinimide ring. The lethal doses of most of the active succinimides were higher than 5000 mg/kg p.o. With sublethal doses mice sometimes become drowsy and had myoclonic seizures and/or diarrhoea. At therapeutic dose levels kinetic disturbances, potentiation of pentobarbitone hypnosis or analgesia were rarely observed.