Complex patterns of intercellular calcium signaling occur in hippocampal slice organotypic cultures from neonatal mice. Spontaneous, localized intercellular Ca2+ waves involving 5-15 cells propagate concentrically from multiple foci in specific cellular layers of the hippocampus (the stratum oriens and stratum radiatum). In these same regions, extensive intercellular Ca2+ waves involving hundreds of cells travel as curvilinear and spiral wavefronts across broad areas of the slice. When two curvilinear wavefronts collide, they annihilate at the point of contact. Intercellular Ca2+ waves travel at rates of 5-10 µm/sec, are abolished by thapsigargin, and do not require extracellular Ca2+. Staining for astrocytes and neurons indicate that these intercellular waves occur primarily in astrocytes. The frequency and amplitude of intercellular Ca2+ waves increase in response to bath application of N-methyl-D-aspartate, and decrease in response to removal of extracellular Ca2+ or application of tetrodotoxin. This novel pattern of intercellular calcium signaling suggests that networks of glial cells in the hippocampus may behave as an excitable medium whose spatial and temporal signaling properties are modulated by neuronal activity.