Mg2+, Ca2+, Mn2+, Zn2+, and Cu content of neurons from chick embryo cortex cultivated in chemically defined serum free growth medium was determined by energy dispersive X-ray fluorescence and atomic absorption spectroscopy. The intracellular volume of cultured neurons was determined to be 2.73 microliters/mg. Intracellular Mn2+, Fe2+, Zn2+, and Cu2+ in the cultivated neurons were 100-200 times the concentrations in the growth medium: Mg2+ and Ca2+ were 0.9 and 1.7 mM respectively, around 20 fold higher than in growth medium. Mg2+, Fe2+, Cu2+ and Zn2+ concentrations in neurons were in the range of ca. 300-600 microM, approximately 2-3 times the values previously reported in glial cells; Ca2+ and Mn2+ content of the neurons were higher by 5 and 10 fold respectively compared to glial cells. In neurons, the subcellular distribution of Fe2+, Cu2+, and Mn2+ follows the rank order: cytosol greater than microsomes greater than mitochondria; for Zn2+ the distribution differs as following: cytosol greater than mitochondria greater than microsomes. Determination of the superoxide dismutase activities in the cultivated neurons indicated that the Mn2+ linked activity predominates whereas, the Cu-Zn dependent enzyme is dominant in glial cells. Enrichment of the culture medium with Mn2+ to 2.5 microM enhanced the Mn-SOD by approximately 33% but the Cu2+-Zn2+ enzyme activity was not modified. The high Mn2+ content, the capacity to accumulate Mn2+, and the predominancy of the Mn-SOD form observed in neurons is in accord with a fundamental functional role for this metal ion in this type of brain cells.