Neurotrophins have been proposed to participate in activity-dependent modifications of neuronal connectivity and synaptic efficacy. Preferential strengthening of active inputs requires restriction of putative neurotrophin-mediated synaptic potentiation to active synapses. Here we report that potentiation of synaptic efficacy by brain-derived neurotrophic factor (BDNF) is greatly facilitated by presynaptic depolarization at developing neuromuscular synapses. Brief depolarization in the presence of low-level BDNF results in a marked potentiation of both evoked and spontaneous synaptic transmission, whereas exposure to either BDNF or depolarization alone is without effect. This potentiation depends on the relative timing of depolarization and reflects an enhancement of transmitter secretion from the presynaptic neuron. Thus synapses made by active inputs may be selectively strengthened by secreted neurotrophins as part of activity-dependent refinement of developing connections or of mature synapses.