Intracellular excitatory postsynaptic potentials (EPSPs) were recorded in area CA1 of hippocampal slices from guinea pigs. With paired-pulse stimulation of stratum radiatum, the second stimulus (interval of 40-50 ms) produced an EPSP with enlarged amplitude. Two methods of quantal analysis showed an increase in quantal content of the facilitated EPSP and a smaller increase in quantal size. The correlation between amplitudes of the first and second EPSP was usually insignificant. The results favour a presynaptic location of the mechanisms of the paired-pulse facilitation and suggest increases in the average of transmitter quanta released by presynaptic volley as well as increases in the amount of transmitter in each quantum.