We revealed the structural features of astrocytes by means of light microscopy, confocal laser scanning microscopy and high voltage electron microscopy, and estimated their numerical densities in the mouse hippocampus. The high voltage electron microscope examinations of Golgi-impregnated astrocytes clearly disclosed their fine leaflet-like processes in the masses occupied by individual astrocytes. The intracellular injection of two different fluorescent tracers into two neighboring astrocytes revealed that each astrocyte occupied a discrete area with a limited overlap only at its peripheral portion. In a quantitative analysis using an optical dissector, the numerical densities of astrocytes identified as S100-immunoreactive cells were only slightly different in their areal and laminar distributions. The numerical densities were higher in the stratum lacunosum-moleculare and dentate hilus, while they were slightly lower in the principal cell layers than the average (24.2 x 10(3) mm(-3)) in whole hippocampal regions. As for the dorsoventral difference, the numerical densities were significantly larger at the ventral level in the dentate gyrus, whereas such tendency was not apparent in the hippocampus proper. The projection area of the astrocytes estimated from Golgi-impregnated samples was roughly in inverse relation to the numerical densities; the areas in the stratum lacunosum-moleculare were somewhat smaller than the other layers, where the numerical densities were high. The present study indicates that astrocytes are distributed rather evenly without any prominent areal or laminar differences and that the individual astrocytes have their own domains; the periphery of the domain of a given astrocyte is interdigitated intricately with the processes of adjacent astrocytes whereas its inner core portion is not penetrated by them.