The morphology of glial cells in the intact rat optic nerve, a central nervous system (CNS) white matter tract, was analysed by filling over 500 macroglial cells intracellularly with horseradish peroxidase (HRP) or Lucifer yellow (LY). Two main cell types were distinguished: fibrous astrocytes and cells presumed to be oligodendrocytes. Intracellularly stained astrocytes were highly complex, with 50-60 long branching processes which passed radially from the cell body and terminated in end-feet at the pial surface or on blood vessels; some processes ended freely in the nerve parenchyma. Astrocytes filled with LY were usually dye-coupled to other astrocytes after the first week of life. Filled oligodendrocytes had a unique appearance that unmistakably distinguished them from astrocytes and were occassionally dye-coupled to nearby oligodendrocytes. These cells had 20-30 longitudinally oriented processes 150-200 microns long, which passed exclusively along the long axis of the nerve parallel to axons; the longitudinal processes were connected to the cell body by thin branches 15-30 microns long. The longitudinal processes probably represent the tongue processes of the internodal myelin sheaths, and thus each oligodendrocyte appears to myelinate 20-30 axons with sheaths that are 150-200 microns in length.