Organotypic slice cultures from rat hippocampal cortex grown in an interface between culture medium and a CO2-enriched atmosphere maintained much of the morphological connectivity characteristic of the hippocampus in situ and thinned out considerably, facilitating optical measurements of fluorescent dyes sensitive to Ca2+ in individual neurons. Pyramidal neurons of the CA3 region presented morphological features of differentiated cells, including complex dendritic arborization and large numbers of dendritic spines. The fine cytoskeletal substructure at the postsynaptic density, below the plasma membrane, and within the core of the head and neck of dendritic spines in rapidly frozen slice cultures presents the characteristic morphology previously described for Purkinje cell dendritic spines in acutely dissected cerebellar cortex slices after rapid freezing. CA3 neurons responded to intracellular current injection with a train of action potentials, spike frequency adaptation, and a slow afterhyperpolarization. These spike trains caused rapid increases in dendritic [Ca2+]i that decayed to resting levels after termination of the current pulse. Dendritic spines were clearly observed in proximal dendrites of CA3 neurons in live preparations. [Ca2+]i transients in these dendritic spines closely followed the changes observed in the main dendritic shaft. Orthodromic synaptic stimulation from the dentate hilus generated long-lasting synaptic potentials accompanied by large [Ca2+]i transient in CA3 pyramidal neurons. The [Ca2+]i response was first observed in the proximal dendrites, after which the soma exhibited a [Ca2+]i increase, returning to resting levels within 10 s after the synaptic stimulus. Slice cultures thus provide a favorable opportunity to investigate [Ca2+]i responses in individual neurons maintained in an organotypic synaptic environment, taking advantage of high-resolution optical techniques. © 1993 Academic Press, Inc.