A readily releasable pool of quanta, tentatively identified with docked synaptic vesicles, has been defined by analysis of the neurotransmitter release caused by application of hypertonic solutions. The goal of this work is to determine the relationship of this functionally defined readily releasable pool to the one drawn upon by action potential-evoked release. We find that hypertonic solutions do not act through changes in intracellular calcium. Since the release produced by action potentials and hypertonic solutions varies in parallel as the pool size is changed, we conclude that the same pool is shared by both mechanisms. This conclusion, taken together with other observations in the literature, means that the synaptic release probability depends on the size of the readily releasable pool.