A number of recent results suggest that neurotrophins play an important role in early development as well as in the later, activity-dependent processes important for the final shaping of cortical connections. Many neurotrophins and their receptors are regulated in parallel with the 'critical period' in development, and their application to the neocortex can dramatically alter the functional organization of the cortex, as well as the morphological properties of neocortical neurons. In addition, recent data show that a different phenomenon of synaptic plasticity, hippocampal long-term potentiation, also critically depends on neurotrophins. Thus, neurotrophins may play a role in linking functional modifications of synapses to the morphological effects of synaptic stabilization and rearrangement, as observed in the neocortex.