Tranquilizer effect of aerobic exercise was studied in seven college students. The seven subjects acted as their own controls in an experimental design where each subject was tested twice before and after exercise and twice before and after control (quiet sitting). Exercise consisted of 20-min cycling on an ergometer with the load adjusted to elicit a heart rate rise of 60% between resting and maximal value. Testing consisted of measurements of resting electroencephalogram (EEG) and spinal reflex excitability (H-reflex). EEG signal was digitized at a sampling frequency of 64 Hz and analyzed by means of computer-aided decomposition algorithm and frequency power spectral analyses, respectively. For the assessment of spinal reflex excitability, electrical stimulation of the tibial nerve in the popliteal fossa was applied to elicit H-reflexes (H waves) and directly propagated waves (M waves). The H/M ratio was then calculated and used as the criterion variable. Results indicated that there was no significaint difference in the H/M ratio on the control day (-0.27%, p>0.05), but showed a dignificant decrease after exercise (-15.9%, p<0.01). EEG spectral analysis demonstrated /significant increase (p<0.05) in relatively low frequency components, e.g. delta (+23.8%) and alpha (+18.1%)waves. These results confirm earlier resting EMG results and further suggest that tranquilizer effect of exercise may reside not only in the peripheral muscle motoneuron excitability, but also in the higher central nervous system.