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1. Whole‐cell recording from pairs of adjacent mouse hippocampal neurons in culture was used to study the quantal properties of action potential‐evoked excitatory synaptic transmission and to demonstrate the use of Sr2+ in quantifying those properties. 2. In the presence of extracellular Sr2+, excitatory postsynaptic currents (EPSCs) were followed by an after‐discharge of miniature excitatory postsynaptic currents (mEPSCs) lasting 1‐2 s and generated by evoked asynchronous release of presynaptic quanta of transmitter. Like the EPSC of which it is thought to be an extension, the after‐discharge was modulated by procedures expected to modulate Sr2+ influx into the nerve terminal. The number of mEPSCs in the after‐discharge was decreased by increasing extracellular [Mg2+], and increased by increasing extracellular [Sr2+] or increasing the number of action potentials used to evoke the after‐discharge. 3 …