Fourteen patients with either cortical seizure, excessive peripheral motor activity, or both received diazepam intravenously during electroencephalographic recording. In ten, simultaneous electromyographic monitoring was performed. The results indicate that intravenously administered diazepam can be used to control excessive motor activity without masking diagnostic features of the EEG. In addition, the drug may have the advantage of suppressing some of the secondary paroxysmal activity in cases of focal seizures with secondary generalized spread. Accentuation of focal slowing and induced asymmetrical low voltage fast activity both have lateralizing value. The literature about the site of the major action of diazepam is reviewed, and evidence for its activity in the mesencephalic reticular formation is stressed.