With a pyroantimonate precipitation technique, we have demonstrated Ca2+ in the sacs of the dendritic spine apparatus in the SER of dendrites and axon terminals, in synaptic vesicles, multivesicular bodies, mitochondria, and glial processes of the dentate molecular layer. It is speculated that the spine apparatus may be a Ca2+ sequestering organelle which may regulate levels of intraspinal and intradendritic Ca2+ during synaptic activity.