Rat brain synaptosomes, prepared by discontinuous Ficoll density gradient centrifugation, accumulated 45Ca during brief incubations in modified Krebs-Ringer media. Uptake of 45Ca was increased by 5 mM glutamate and 50 mM KCl, conditions that depolarize nerve cells; uptake of 22Na was also increased by these agents. With 0.2 mM diphenylhydantoin, the increased 45Ca uptake due to KCl was diminished, whereas that due to glutamate was less affected; conversely, with 0.15 muM tetrodotoxin the increased 45Ca uptake due to glutamate was diminished, whereas that due to KCl was less affected. Both diphenylhydantoin and tetrodotoxin diminished the augmented uptake of 22Na due to KCl and glutamate; thus the increased uptake of 45Ca under depolarizing conditions may be dissociated from the increased influx of sodium. Ruthenium red decreased the uptake of 45Ca under all conditions, as did procaine and the lanthanide Pr3+. Neither 5 mM glutamate nor 50 mM KCl increased 45Ca uptake by brain mitochondria under comparable experimental conditions, whereas ATP increased the uptake by mitochondria but not that by these synaptosomes. Altering the sodium gradient by equimolar substitution of lithium or choline for sodium in the medium increased 45Ca uptake, whereas 22Na uptake was decreased. Inhibiting the sodium pump by ouabain or strophanthidin also increased 45Ca uptake, and increased 22Na uptake as well. The increased uptake of 45Ca induced by ouabain was inhibited by diphenylhydantoin and tetrodotoxin. Measurements of the total calcium content showed that conditions producing an increased uptake of 45Ca also produced a net uptake of calcium, rather than merely accelerating a 45Ca-40Ca exchange. Experiments measuring the loss of previously accumulated 45Ca showed that directly decreasing the sodium gradient or inhibiting the sodium pump slowed the loss of 45Ca. These data are considered in terms of calcium influx through 'leak' pathways and gated channels (sensitive to membrane depolarization) and of net efflux dependent on a coupled sodium-calcium exchange mechanism.