Reactive astrocytes are thought to be impediments to axon regrowth following injury to the mammalian central nervous system. Axon and dendrite growth, however, appear to be differently controlled by normal astrocytes in vitro. To determine whether reactive astrocytes differ in their ability to support axons or dendrites, embryonic (E18) mouse cortical neurons were cocultured with P4 and P12 rat astroglial cells derived from normal or lesioned cortex. After 5 days in vitro, axon and dendrite outgrowth from isolated neurons was quantified with double-labeling immunohistochemical techniques. Reactive astrocytes were able to maintain primary dendrite growth, principally primary dendrite number. Axon elongation, however, was significantly reduced on both neonatal and more mature reactive astrocytes. These results indicate that reactive astrocytes may exhibit separate mechanisms to control dendrite and axon growth.