The electroencephalographic (EEG) activity was studied in ICR mice. EEG patterns obtained by bipolar leads from the mice were classified into five stages: awake-exciting (AE), awake-resting (AR), drowsy (DR), deep sleeping (DS) and REM sleeping (RS) stage. These stages were characterized by the following EEG patterns. AE: Rhythmic θ-activity of 4 to 6 Hz accompanied with an irregular low voltage and fast waves appeared continuously. AR: High voltage and slow waves (HVSWs) lasting for a few seconds appeared at intervals of 3 to 10 seconds. DR: Spindles with a duration of 1 to 5 seconds were frequently observed at intervals of 3 to 5 seconds. DS: Bursts of HVSWs and spindles were reciprocally repeated with durations of 5 to 10 and 10 to 30 seconds, respectively. RS: The θ-activity of 6 to 8 Hz lasting for 2 to 250 seconds appeared with a regular rhythm. The five stages were interchanged within such a short time that the cyclic occurrence of the RS stage was shown at intervals of about 20 seconds to 10 minutes. From these results it was concluded that the electrical characteristic of the brain of the mouse was the conspicuous appearance of θ-activity and spindles, and that the EEG stages alternated frequently with one another in this animal.