Neuron-glia cocultures were prepared using, as a source for glial cells, either C6 glia (2B clone) of early (2B23) or late (2B111) passages or advanced passages of glial cells derived from primary cultures prepared from aged mouse cerebral hemispheres (MACH). Six-day-old chick embryo cerebral hemispheres (E6CH) were the source of neuron-enriched cultures. Glutamine synthetase (GS) activity was used as a marker for astrocytes and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) activity was used as a marker for oligodendrocytes. GS activity was markedly enhanced in cocultures of E6CH neurons and 2B23 glioblastic cells, whereas GS activity was reduced in cocultures of E6CH neurons and 2B111 astrocytic glia. In contrast, CNP activity was enhanced in cocultures of C6 glial cells with E6CH neurons. Glial cells from aged mouse brain did not respond to coculturing with E6CH neurons. It appears from these findings that neuronal input enhances the differentiation of glioblastic cells to either astrocytic or oligodendrocytic expression, whereas it decreases the activity of committed astrocytes. In contrast, glial cells from aged mouse brain do not respond to neuronal input. Choline acetyltransferase (ChAT) activity, a marker for cholinergic neurons, was enhanced only when E6CH cultures were grown in conditioned medium (CM) from 2B23 glioblastic cells. In contrast, ChAT activity was markedly diminished when E6CH neurons were cocultured with MACH glial cells but not when grown in CM from MACH glial cells. Thus, humoral factors from immature glial cells appear to enhance cholinergic neuronal phenotypic expression whereas cell-cell membrane contacts with aged glial cells diminish cholinergic phenotypic expression. The findings present supportive evidence that neuron-glia interrelationships are age dependent.