Abstract—The accumulation of calcium ions by brain mitochondria and microsomes and by fractions containing neuronal or glial cells has been studiedin vitrowith techniques involving45Ca and ultramicro‐flame photometry. ATP and substrate‐supported calcium accumulation by brain mitochondria was of the same magnitude as for mitochondria from other organs. Brain microsomes accumulated calcium approximately 15 times less than brain mitochondria. Variations in Na+/K+ratios and in ATP/ADP ratios had a more marked influence on microsomal uptake than on mitochondrial uptake. The passive Ca2+binding by glial cells was higher than neuronal perikarya and synaptosomes. Also the calcium accumulation ability in cell suspensions was slightly higher for glial cells as compared to neuronal perikarya. The calcium uptake by glial cells was stimulated by high external K+concentration, which also was the case for nerve endings. The uptake in neuronal perikarya was unaffected by variations in K+concentration. A comparison between neuronal and glial mitochondria showed that both reach a steady state level of similar magnitude, but that the rate of initial accumulation was greater for glial mitochondria. A high glial calcium accumulation was also observed for the microsomal fra