The purpose of this study is to observe the changes of the values of Met-enkephalin in variolus brain areas of rats, after exhaustive running on the treadmill by means of Radioimmunoussy (RIA). The resultsdemonstrated tbat the values of Met-enkephalin in the artas of cerebral cortex, caudatum. cerebellum andhypothalamus increased, significantly, while there was no significant change in those in areas of thalamus,brain stem and hippocampus. The following. conclusion has been drawn by the authors. The muscular activity is a type of stressorwhich can not only cause the changes of the values of ?-endorphin in plasma, but also cause the increase ofMet-enkephalin released from its neuron. Further investigations are needed on the problem whether the inc-rease in Met-enkephalin may have Dositive or negative physiological effects on the muscular activity of livingbody.