In Drosophila, mutations in specific ion channel genes can increase or decrease the level of neural/synaptic activity. We have used these genetic tools, in combination with classical pharmacological agents, to modulate neural activity during embryogenesis and examined effects on the differentiation of an identified neuromuscular junction. We find that electrical activity is required for the neural induction of transmitter receptor expression during synaptogenesis. Likewise, neural electrical activity is required to localize transmitter receptors to the synaptic site. In muscles with activity-blocked synapses, a low level of receptors is expressed homogeneously in the muscle membrane as in muscles developing without innervation. Thus, presynaptic electrical activity is required to mediate the neural induction of the transmitter receptor field in the postsynaptic membrane.