Evidence is emerging to suggest that in addition to their "classical" neurotrophic involvement in the regulation of the differentiation, maturation and survival of neurons, neurotrophins play crucial roles in neural transmission and succeeding activity-dependent plasticity of synapses. Here we discuss: 1) the regulated synthesis and secretion of neurotrophins in response to neural activity, 2) the short- and long-term effects of neurotrophins on neural transmission, and 3) the neurotrophin-induced rearrangement of synaptic networks.