Quantal components of Schaffer collateral synaptic transmission recorded intracellularly from CA1 pyramidal cells were examined using 2 methods: simultaneous recordings of CA3-CA1 cell-pairs, and minimal electrical stimulation in stratum radiatum. Quantal parameters estimated by the method of failures and by a computer algorithm that optimized parameter estimates using deconvolution of background noise were highly correlated. EPSP-amplitude histograms of CA3-CA1 cell pairs (N = 10) and minimal electrical stimulation (N = 33) could be adequately described either by Poisson or binomial statistics, or by both, and exhibited similar estimates of unit quantal size (q) and mean quantal content (m). Paired-pulse stimulation with 50 msec between stimuli resulted in an expected facilitation in the EPSP amplitude and increase in m during the second response, as estimated by noise deconvolution, by the decrease in apparent failures, and by a decrease in the coefficient of variation of the EPSP. Tetanization of the Schaffer collaterals that induced long-term enhancement (LTE/LTP) of the population response was associated with an average increase in q for minimal-stimulation responses, with no significant change in any estimate of m. Taken together, these data indicate that, under the present experimental conditions, LTE is expressed as an increase in quantal size, rather than an increase in the number of quanta released per presynaptic impulse. Although this is not definitive evidence for a postsynaptic mechanism, these findings do further restrict the classes of possible presynaptic mechanisms that may be proposed to account for LTE expression.