We have used endocytotic uptake of the styryl dye FM1-43 at synaptic terminals (Betz and Bewick, 1992) to study properties of individual synapses formed by axons of single hippocampal neurons in tissue culture. The distribution of values for probability of evoked transmitter release p estimated by dye uptake is continuous, with a preponderance of low p synapses and a broad spread of probabilities. We have validated this method by demonstrating that the optically estimated distribution of p at autapses in single-neuron microislands predicts, with no free parameters, the rate of blocking of NMDA responses by the noncompetitive antagonist MK-801 at the same synapses. Different synapses made by a single axon exhibited varying amounts of paired-pulse modulation; synapses with low p tended to be facilitated more than those with high p. The increment in release probability produced by increasing external calcium ion concentration also depended on a synapse's initial p value. The size of the recycling pool of vesicles was strongly correlated with p as well, suggesting that synapses with higher release probabilities had more vesicles. Finally, p values of neighboring synapses were correlated, indicating local interactions in the dendrite or axon, or both.