Complex patterns of intercellular calcium signaling occur in the CA1 and CA2 regions of hippocampal slice organotypic cultures from neonatal mice. Spontaneous localized intercellular Ca2+ waves involving 5-15 cells propagate concentrically from multiple foci in the stratum oriens and s. radiatum. In these same regions, extensive Ca2+ waves involving hundreds of cells propagate as curvilinear and spiral wavefronts across broad areas of CA1 and CA2. Ca2+ waves travel at rates of 5-10 mu m/s, are abolished by thapsigargin, and do not require extracellular Ca2+. Staining for astrocytes and neurons indicates that these intercellular waves occur primarily in astrocytes. The frequency and amplitude of Ca2+ waves increase in response to bath application of N-methyl-D-aspartate (NMDA) and decrease in response to removal of extracellular Ca2+ or application of tetrodotoxin. This novel pattern of intercellular Ca2+ signaling is characteristic of the behavior of an excitable medium. Networks of glial cells in the hippocampus may behave as an excitable medium whose spatial and temporal signaling properties are modulated by neuronal activity.