Synchronous neurotransmitter release is a highly regulated process that takes place at specializations at the presynaptic membrane called active zones (AZs). The relationships between AZs, quantal release, and vesicle replenishment are not well understood in a mature synapse. We have measured the number, distribution, and other properties of AZs in mouse motor nerve terminals and combined these observations with electrophysiological estimates of the size of the readily releasable pool (RRP) of synaptic vesicles. On average, we counted 850 AZs per terminal. Assuming two primary docked vesicles per AZ, we predict a total of ∼1700 vesicles optimally positioned for exocytosis. Electrophysiological estimates of the size of the RRP, using a simple kinetic model that assumes exponential depletion of the initial pool and refilling by recruitment, gave an average value of 1730 quanta during 100 Hz stimulation, in satisfying agreement with the morphology. At lower stimulus frequencies, however, the model revealed that the estimated RRP size is smaller, suggesting that not all AZs participate in release at low stimulation frequencies.