Electron-microscopic evidence indicates that during conditions of high hormone demand such as lactation there is a dramatic reduction in the number of fine glial processes which are normally interposed between magnocellular neuroendocrine cell somata in the supraoptic nucleus (SON). The purpose of this study was to corroborate these data at the light-microscopic level and to gain some insight into what underlying events might accompany these apparent morphological changes. The distribution of the glial fibrillary acidic protein (GFAP), an intermediate filament component of the astrocytic cytoskeleton, was visualized in lactating or estrous rats using peroxidase-antiperoxidae immunocytochemistry. Computerized image analysis was employed to determine and compare the staining distributions of this protein for the two groups of rats. Statistical analysis revealed a redistribution of GFAP immunostaining in the SONs of lactating animals as compared to controls. No differences in staining were found in a control area dorsolateral to SON. The pattern of change was to a less dense, more homogeneous distribution of GFAP in lactating rats, a change which could be interpreted as reflective of a reduction in the number of densely staining glial processes. © 1985 Springer-Verlag.