Intracellular Ca2+ dynamics have been measured using imaging techniques in dendrites and spines of CA3 hippocampal neurons in brain slice under both acute and tissue culture conditions. In response to presynaptic stimulation, micromolar levels of Ca2+ are rapidly reached in spines of distal dendrites. If stimulus parameters are chosen judiciously so as to minimize postsynaptic firing, then the dendrite shaft increases are far less. Spine Ca2+ increases are largely dependent upon activation of NMDA receptors. At the large mossy fiber synapses, presynaptic stimuli also produce large Ca2+ increases but the differences in shaft-spine Ca2+ levels are much less; often they are insignificant. Also at these locations, postsynaptic firing, without presynaptic stimulation is sufficient to produce large increases in spine Ca2+ levels.