1. Quantal release from single hippocampal glutamatergic (CA3-CA1) synapses was examined in the neonatal rat during a 10 impulse, 50 Hz stimulus train. These synapses contain a single release site only, thus allowing for an analysis of frequency facilitation/depression at the single release site level. 2. These synapses displayed a considerable heterogeneity with respect to short-term synaptic dynamics, from a pronounced facilitation to a pronounced depression. Facilitation/depression was the same whether evaluated using the magnitude or the probability of occurrence of the postsynaptic response. This result suggests that postsynaptic factors, such as desensitisation, play little role. 3. Release probabilities initially and late during the train were uncorrelated. Initially, release is determined by the number of immediately release-ready vesicles and by the probability of releasing such vesicles (P(ves)). Within the first five stimuli this vesicle pool is depleted. The deciding factor for release is thereafter the rate at which new vesicles can be recruited for release, rather than P(ves). 4. Heterogeneity in facilitation/depression among the synapses was strongly correlated with heterogeneity in initial P(ves) but not with that of the immediately release-ready vesicle pool. Thus, the main factors deciding short-term synaptic dynamics are heterogeneity in initial P(ves) and in vesicle recruitment rate among the synapses.