Quantitative EEG is a sensitive method used to assess the effects of pharmacological substances on the central nervous system (CNS) activity. A standard technique is to measure the EEG under vigilance-controlled and resting conditions for a short duration, for example 5 min. The aim of the present study was to investigate the stability of 5-min EEG recordings. While the time course of the EEG was fairly stable during the recording session under the vigilance-controlled condition, systematic trends became apparent under the resting condition. Pharmaco-sensitivity of the EEG and its reliability increased with the recording duration. Five minutes of EEG recording seem to be sufficient and well chosen to evaluate the influence of drugs on the EEG.