The work is to study possible mechanisms of radiation injuries to brain by observing Ca,2+, and Mg,2+, contents in electron beam irradiated rat brain. Sprague-Dawley rats were irradiated with a single dose of 10Gy, 20Gy or 30Gy by 6MeV electrons. On different days after the irradiation, Ca,2+, and Mg,2+, contents in whole brain of the rats were measured by direct-reading plasma spectrography, and the brain water content was calculated with wet-dry weight formula. In comparison to the control groups, the total tissue water and Ca,2+, content in brain of all the irradiated rats increased, while the Mg,2+, content in brain decreased. The of Ca,2+, content increased markedly (,p,<,0.05) 24h after the irradiation, and the Mg,2+, content decreased significantly (,p,<,0.05) 7d after the irradiation. The content changes became severe with increasing dose. Discusions are made on mechanisms of radiation induced brain injuries based on observations of the Ca,2+, and Mg,2+, contents changes in the rat brain.