Temporal spike correlation in pre- and post-synaptic cells strengthens or weakens synapses in development or in cellular models of learning (long-term potentiation and depression), two well-known paradigms being Hebb’s postulate and spike-timing-dependent plasticity. A favorable model to investigate synaptic modification is the input elimination that occurs at developing neuromuscular junctions as a result of competition between the motor nerve terminals. Activity influences this process, but its precise role remains controversial. Here we present a series of studies in which we address the role of spike timing in the competing inputs: we provide evidence that synchronous activity blunts competition among motor nerve terminals while an asynchronous one strongly activates competition leading to synapse elimination.