About phenytoinhydantoin derivative, anticonvulsant, class ib antiarrhythmic agent.mechanism of action of phenytoinphenytoin acts on the motor cortex where it stabilizes neuronal membrane and inhibits the spread of seizure discharge. it prolongs the inactivated state of voltage sensitive neuronal sodium ion channel either by increasing the efflux or decreasing the influx of sodium ion across the cell membrane and inhibits the high frequency discharge. antiarrhythmic action of phenytoin is due to its membrane stabilizing effect and blockage of sodium channels. it causes depression of automaticity in ventricular and purkinje fibres. at high concentrations phenytoin 1. enhances calcium binding to phospholipids in neuronal membrane and results in a more stable neuronal membrane. 2. it enhances the concentration of inhibitory neurotransmitter gaba 3. it inhibits the excitatory glutamate receptors. all these indicate that phenytoin limits the development of maximal seizure