Records of spontaneous activity from units likely to be radial afferents were analyzed to find the origin of spontaneous action potentials in single auditory nerve fibers. Single synaptic events (excitatory postsynaptic potentials or EPSPs) nearly all triggered action potentials (spikes). An abrupt increase in slope during the rising phase of the EPSP often signalled the initiation of an action potential. Synaptic potentials that did not trigger spikes occurred frequently during the refractory period. These events sometimes appeared to be composed of subunits. Refractoriness appears to be the primary reason these EPSPs were ineffective. Distributions of the onset slopes of postsynaptic potentials were highly skewed. Skewing was not a consequence of refractoriness, but most likely because the amplitude distribution of spontaneous potentials is not gaussian.