Introduction: Stress is a main factor influencing brain functions as revealed by the electroencephalogram (EEG) recordings. Moreover, different stress durations seemingly cause perturbations in brain waves and lead to mental disorders. This study investigates the effects of acute, sub-chronic and chronic stress on EEG in rats.  Methods: Twenty-eight Wistar adult male rats were randomly allocated to one control and three experimental groups subjected to 6 hr/day of acute (1d), sub-chronic (7d) and chronic (21d) stress. At the end of each period, 20 minutes of EEG recording was taken of each subject. Results: Percentages of delta, theta and alpha frequencies of the baseline in the chronic stress group showed significant differences from those of the control (P<0.05). Theta waves increased in the chronic stress group compared to the acute and sub-chronic (P<0.05 and P<0.01; respectively) ones. This is while, compared to the control, the acute and sub-chronic stress groups exhibited significantly increased percentages of beta waves (P<0.05 in both). Conclusion: The data indicate that different stress durations have different impacts on the EEG rhythm. Acute and sub-chronic stress durations led to changed cortical activity, indicating the inability of the subjects to cope with the stress imposed. Also, chronic stress caused irregularities in the EEG rhythm (delta, theta and alpha waves). EEG recording seems to be useful for measuring stress levels and for predicting abnormalities due to different stress durations.