Developmental changes in rat hippocampal transmitter release and synaptic plasticity were investigated. Recordings from pairs of pyramidal neurons in slices showed that an action potential in a CA3 neuron released only a single quantum of transmitter onto a CAI neuron. Failures of synaptic transmission reflected probabilistic transmitter release. The probability of release (P-r) was 0.9 in 4- to 8-day-old rats and decreased to less than 0.5 at 2 to 3 weeks. Long-term potentiation (LTP) in 2- to 3-week-old rats was associated with an increase in P-r from a single synaptic site. The high initial P-r in 4- to 8-day-old rats normally occludes the expression of LTP at this stage.