sr2 and quantal events at excitatory synapses between mouse hippocampal neurons in culture 1 wholecell recording from pairs of adjacent mouse hippocampal neurons in culture was used to study the quantal properties of action potentialevoked 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 afterdischarge of miniature excitatory postsynaptic currents mepscs lasting 12 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 afterdischarge was modulated by procedures expected to modulate sr2 influx into the nerve terminal the number of mepscs in the afterdischarge was decreased by increasing extracellular mg2 and increased by increasing extracellular sr2 or increasing the number of action potentials used to evoke the afterdischarge 3 epscs recorded in media containing either 1 mm ca2 or 6 mm sr2 were of similar amplitude adding sr2 to lowca2 media increased epsc amplitude while adding sr2 to highca2 media lowered epsc amplitude these results suggest that extracellular sr2 is less effective than ca2 in supporting quantal release 4 the levels of extracellular ca2 mg2 and sr2 were adjusted so that most afterdischarge mepscs were discrete and comparable in numbers to the quantal events that contributed to the corresponding evoked epscs in a series of twentyfive pairs of neurons the mean amplitude of mepscs recorded at 80 mv was 35 10 pa and the mean coefficient of variation was 050 010 range 026062 the mepsc amplitude histogram was positively skewed 5 in ten pairs of neurons the mean and variance of epscs and mepscs and quantal content were determined from samples of more than 100 evoked events in superfusion solutions containing mm 05 ca2 2 sr2 and 10 mg2 and mean quantal content was determined from the ratio of amplitudes of the mean epsc and mepsc a binomial quantal analysis produced values of 212 for napp apparent number of independent synapses and 025075 for papp apparent probability of releasing a quantum at one of those synapses these parameters predicted the number of observed failures the observed coefficient of variation for quantal content predicted the observed coefficient of variation of the epsc amplitude when the coefficient of variability of quantal amplitude of afterdischarge mepscs was taken into account 6 in six pairs of neurons where more than 250 evoked events were recorded the observed amplitude histogram for epscs could be approximated by a predicted amplitude distribution generated from the estimated binomial parameters and an empirical function describing the amplitude distribution of afterdischarge mepscs 7 the observation that parameters derived from mepscs that contribute to the sr2generated afterdischarge can predict the shape of the epsc amplitude distribution and a quantal content consistent with the observed failure rate and epsc amplitude variance suggests that this subset of mepscs has the same properties as the quantal events released around the time of the peak of the corresponding