The quantal description of synaptic transmission, first formulated at the neuromuscular junction, has recently been extended to the Schaffer collateral-CA1 synapse of the mammalian hippocampus. In a subset of connections, an upper limit can be placed on the coefficient of variation of the quantal amplitude, on occasion lower than 0.2. In the same connections, the average quantal amplitude implies that fewer than 20 fast glutamate-gated ion channels open to carry the quantal current. With so few channels opening, their stochastic behaviour imposes a minimal trial-to-trial variability on the quantal current. These observations can be reconciled by postulating that most of the available receptors in the synaptic cleft are bound by released transmitter and that ionophores open with a high probability.