Diphenylhydantoin (DPH) kinetics were studied in five male patients with poorly controlled seizures. EEG recordings were taken on four of the patients immediately prior to and 30 minutes following a 60 minute i.v. infusion of phenytoin sodium. Mean phenytoin levels following post-infusion EEG recordings were 17.1 mg/L. Spectral analysis revealed that phenytoin produced a decrease in the variance of EEG spectral power as a function of time in frequencies between 8-18 Hz. This decrease in the variance of the time series of EEG spectra suggests increased EEG stability and may represent an electrographic marker of the anticonvulsant action of phenytoin.