1. Long-term potentiation (LTP) was induced in EPSCs evoked in CA1 pyramidal neurones of young rats in vitro by extracellular stimulation of stratum radiatum. Low frequency stimulation was paired with postsynaptic depolarization to induce LTP, using whole-cell recording techniques. 2. Sufficient control and potentiated records were obtained under stable recording conditions to allow a quantal analysis of eleven EPSCs. The fluctuations in amplitude of all eleven EPSCs were quantized before conditioning stimulation, and they remained quantized after LTP induction, usually with an increased quantal variance. 3. Quantal current was increased by conditioning for nine out of eleven EPSCs. The increase in quantal current was correlated with the percentage increase in the EPSC. For only two EPSCs could the entire potentiation be attributed to an increase in quantal current. 4. The amplitude fluctuations of five control EPSCs could be described by binomial statistics, but after conditioning the binomial description held for only one of these EPSCs. For this EPSC, conditioning caused the release probability to increase from 0.39 +/- 0.05 to 0.47 +/- 0.02. 5. Quantal content was increased by conditioning stimulation for ten out of eleven EPSCs. The increase in quantal content was correlated with the percentage increase in the EPSC. However, for only four EPSCs could the entire potentiation be attributed to an increase in quantal content. 6. Most EPSCs were evoked with a high proportion of response failures. The probability of response failures decreased in eight out of eleven EPSCs following the induction of LTP. There was a negative correlation between the change in the probability of response failures and the amount of LTP. 7. The minimal number of sites at which transmission occurred increased for ten out of eleven EPSCs following LTP induction. Increases in the minimal number of active sites following conditioning were associated with decreases in the probability of response failures for seven out of eleven EPSCs. 8. The induction of LTP usually resulted in changes in the time course of the EPSCs. Cable analysis using a passive compartmental model of a CA1 pyramidal cell suggested that these time course changes were associated with shifts in the average electrotonic location of the active sites following LTP induction, rather than being caused by an increased duration of synaptic current. 9. LTP expression involves postsynaptic modifications to enhance the synaptic current at active sites. New sites are recruited, and our data cannot be used to determine if this is a result of a pre- or a postsynaptic change. Evidence for an increase in release probability was found for one EPSC.