The ultrastructural localization of calcium deposits in the synapses of rat hippocampus after 10 min global cerebral ischemia was evaluated. Oxalate-pyroantimonate technique was applied. After 24 hours of postischemic recirculation enhancement of intracellular (pre- and postsynaptic parts) and extracellular (synaptic clefts) calcium deposits was found in great proportion of synapses in CA1 sector. Abundant Ca-precipitates appeared specially in synaptics clefts and in the postsynaptic parts near synaptic densities. Increased calcium deposits in some changed mitochondria were also observed, The results presented in this paper suggest synaptic modulation of calcium homeostasis, disturbed after ischemic incident. Presence of Ca-precipitates in synaptic clefts and postsynaptic parts seems to be a sensitive indicator of increased calcium influx from the extracellular to the intracellular compartments.