Abstract: The cytosolic free calcium concentration Ca²⁺i in synaptosomes was determined with the fluorescent indicator fura-2, the effects of calcium channel agonist and antagonist on intracellular Ca²⁺ level were studied. The cytosolic ionized calcium concentrations in resting status were between 200 nmol/L and 400 nmol/L. Cytosolic Ca²⁺ was elevated following increases in Ca²⁺ concentration in the medium, plasma membrane depolarizations induced by KC1, and the addition of glutamate and NMDA. On the other hand, the increase of cytosolic Ca²⁺ induced by KCI was decreased by verapamil and that induced by NMDA was decreased by MgCl₂.A few critical problems in Ca²⁺i detection were also discussed.