Ultradian variations of interhemispheric activities were studied on 8 right-handed male subjects. Parietal EEG (P3, P4) was recorded every 30 min for 1-min duration from 08:00 to 19:00 in eyes closed rest periods. EEG laterality index scores (LISs) were calculated from the amplitude (square root of power) of alpha (8-13Hz). LISs showed significant periodicities in all subjects, indicating EEG asymmetries fluctuate with time. Moreover, the periods and acrophases of the observed ultradian variations were different among the subjects. The relationships between the EEG asymmetry and the cognitive functions were discussed.