The effect of brain-derived basic fibroblast growth factor (bFGF) on the membrane structure of cultured rat astroglial cells was investigated by quantitative freeze-fracture replica examination. In the presence of bFGF the number of the orthogonal arrays of particles (OAP), which are characteristic compounds of the membrane of mammalian astrocytes, is clearly reduced. This finding is discussed in the framework on current views about neuro-glial interactions and their implications in the process of fiber regeneration in the central nervous system.