A growing body of literature has documented the differential role of the frontal regions of the two cerebral hemispheres in certain positive and negative affective processes. This corpus of evidence has led to the hypothesis of a possible differential effect of diazepam on asymmetry of frontal activation. To examine this question, nine infant rhesus monkeys were tested on two occasions during which brain electrical activity was recorded from left and right frontal and parietal scalp regions. During one session, recordings were obtained under a baseline restraint condition and then after an injection of diazepam (1 mg/kg). In the other session, following the same baseline restraint condition, a vehicle injection was given. In response to diazepam, the animals showed an asymmetrical decrease in power in the 4-8 Hz frequency band, which was most pronounced in the left frontal region. No change in electroencephalogram (EEG) activity was observed in response to vehicle. Asymmetry in parietal EEG activity was also unchanged by diazepam. Diazepam also produced overall reductions in power across different frequency bands in both frontal and parietal regions. Good test-retest stability of EEG measures of activation asymmetry was also found between the two testing sessions separated by three months. The possible proximal cause of the asymmetrical change in frontal brain electrical activity in response to diazepam, as well as the implications of these findings for understanding the mechanism of action of benzodiazepines are discussed.