Neural circuits rely upon a precise wiring of their component neurons to perform meaningful computations. To compute the direction of motion in the visual scene, the direction selective circuit in the mouse retina depends on an asymmetry in the inhibitory neurotransmission from starburst amacrine cells (SACs) to direction selective ganglion cells (DSGCs). Specifically, depolarization of a SAC on the null side of a DSGC causes a threefold greater unitary inhibitory conductance than depolarization of a SAC on the preferred side. This asymmetry emerges during the second postnatal week of development, but its basis remains unknown. To determine the source of this asymmetry in inhibitory conductance, we conducted paired recordings between SACs and DSGCs at the beginning and end of the second postnatal week. We replaced calcium with strontium to promote asynchronous neurotransmitter release and produce quantal events. During the second postnatal week the quantal frequency but not the quantal amplitude of synapti...