Many growth factors that have effects on neurons are present in the developing and mature central nervous system where their functions are poorly understood. In the peripheral nervous system, target-derived growth factors can regulate the survival of developing afferent neurons. Recent studies suggest that neurotrophins derived from target neurons may also regulate the survival of afferent basal forebrain cholinergic neurons during development of the central nervous system. However, in mature animals, these cholinergic neurons do not appear to require target-derived neuroptrophins for survival. Similar findings have also been reported for dopaminergic neurons of the substantia nigra. Although further studies are required, available experimental evidence suggests that target-derived growth factors may influence neuronal phenotypes such as axonal sprouting or transmitter production instead of survival in mature animals.