The acute effects of a single subcutaneous (sc) administration of methyl bromide (MB) on electroencephalographic (EEG) activity and on sleep-wakefulness and its circadian rhythms were investigated in rats with indwelling electrodes for EEG, electromyographic (EMG) and electrooculographic (EOG) recordings. The LD₅₀ of MB was found to be 135 mg/kg for the sc administration. Slowing of the EEG frequency in the wakefulness (W) stage and spike-wave activity appeared at a dose of 135 mg MB/kg. These abnormal EEG activities did not occur at lower dose levels. Administration of MB at doses of 45, 15 and 5 mg/kg produced dose-related changes in amounts of W, non-REM sleep (NREMS) and REM sleep (REMS) and in their circadian rhythms. Logarithmic concentrations of bromine in blood and brain decreased linearly after the MB administrations, and the half-life of bromine in blood and brain was approximately 8.7 and 4.3 days, respectively. Pretreatment with glutathione effectively lessened the detrimental effects of MB on sleep-wakefulness and its circadian rhythms and increased the LD₅₀.