All synapses exhibit both evoked and spontaneous release ('minis') of neurotransmitters. Recent data have indicated that these two release modes involve distinct mechanisms, but the details are unclear. Using calcium imaging at the Drosophila neuromuscular junction (NMJ), this paper showed that most NMJs exhibiting spontaneous release did not exhibit evoked release, and vice versa. Furthermore, the two modes of release involved distinct sets of NMJ glutamate receptors and were associated with differing levels of the synaptic protein BrP. BrP might therefore be involved in the preference of a synapse for one mode of transmission over another.