Na+, K+, Mg2+, Cu2+, Zn2+, and Mn2+ were determined in several regions of rat brain, using atomic absorption spectroscopy. Cu2+ was highest in the hypothalamus and lowest in the medulla oblongata. Zn2+ was also low in the medulla oblongata and highest in the hippocampal region. Mn2+ was found in high concentration in the hypothalamus.