Primary cultures from fetal rat hypothalamus (embryonic day 15-16) were grown for 9 days in a serum-free medium and then fixed and immunostained for glial fibrillary acidic protein (GFAP). The majority of the GFAP-immunoreactive astrocytes were flat, polygonal, without processes and showed a low intensity immunoreactivity which was restricted to the perinuclear region. Elongated, process-bearing astrocytes, with an intense immunoreactivity in the soma and processes, were also observed in a smaller proportion (30%). Addition of estradiol (10(-12) to 10(-8) M) to the culture medium 24 h before GFAP immunostaining resulted in an increased proportion (59-69%) of process-bearing, intense immunoreactive cells. This effect was blocked by tamoxifen (10(-8) M). The total number of GFAP-immunoreactive astrocytes was not modified by estrogen or tamoxifen. These results indicate that estradiol may modulate the cell shape and the distribution of GFAP in astrocytes in culture and suggest that astrocytes can be a target for sex steroids during development of the central nervous system.