TEMPORAL correlation between pre- and postsynaptic activities is an important mechanism that regulates synaptic connectivity during development and synaptic plasticity in the adult(1-3). In developing neuromuscular junctions, postsynaptic activity is critical in functional suppression and, ultimately, elimination of the synapses(4-6). Although repetitive postsynaptic firing asynchronous to the presynaptic activity results in a persistent synaptic suppression(7-10), the underlying molecular mechanism remains unknown. Here we provide evidence that nitric oxide (NO), a free radical implicated in several forms of synaptic plasticity(11-15), may serve as a retrograde signal for activity-dependent suppression in the neuromuscular synapse. NO donors and activators of the cyclic GMP pathway suppressed spontaneous and evoked synaptic currents. Moreover, the synaptic suppression induced by repetitive postsynaptic depolarization was prevented by the NO-binding protein haemoglobin and by inhibitors of NO synthase. Thus, synaptic suppression may be triggered by NO released from a postsynaptic myocyte that fires asynchronously to the presynaptic terminal.