Although astroglial scar formation is a common response to almost any type of injury to the adult central nervous system, lesions in fetal and neonatal rats have been reported to induce little or no scar formation. To examine this developmental difference further, rats ranging in age from 1 to 65 days postnatal were unilaterally enucleated, a surgical procedure that causes the axons in the optic nerve to degenerate. The optic nerves were processed for light and electron microscopy at times ranging from 7 to 365 days postenucleation. Pronounced and permanent glial scars were formed in every age group examined, including the neonates. However, the time course for removal of the degenerating axonal debris and formation of a compact, debris-free glial scar varied as a function of developmental age. In neonatal rats, a compact glial scar formed in 1-2 weeks whereas 3-5 months were required for compact glial scar formation in juveniles and adults. Changes in cross-sectional area were also associated with optic nerve degeneration and glial scar formation. Whereas lesioned neonatal optic nerves underwent little change in area, there was a substantial decrease in area in the juvenile and adult. Morphometric analysis showed that irrespective of the age of the animal at the time of enucleation, the final area of the compact glial scar was 10-20% of the unlesioned adult control. These results suggest that conflict in the literature over the ability of neonatal astrocytes to form a glial scar may be due to the nature of the lesion or the method of detection since astrocytes in the neonatal rat optic nerve clearly have the capacity to become reactive and form a glial scar.