The serotonin (5-HT)-induced synaptic plasticity of Aplysia sensorimotor synapses has typically been probed by firing a single presynaptic spike. In the present study, 5-HT-induced synaptic plasticity was probed with brief bursts of spikes (10 Hz, 1 sec), which are more behaviorally relevant stimuli. Because such bursts provide a greater challenge to the release machinery than single spikes, their use may reveal additional aspects of synaptic modulation, and, in particular, the role of extracellular signal-regulated protein kinase (ERK), which has recently been implicated in several examples of short- and long-term synaptic plasticity. EPSCs were characterized by their amplitudes. In addition, two kinetic measurements, time to peak and decay time constant, were also determined for the initial EPSC of each burst. Application of 5-HT produced a uniform increase in gain by facilitating each EPSC elicited during a burst of spikes without affecting the kinetics of the initial or last EPSC. These data suggest that short-term facilitation during a burst is mediated largely by processes such as those that affect the size of the releasable pool or rate of vesicle mobilization rather than by an increase in the duration of the presynaptic action potential. An ERK cascade inhibitor (U0126) had no effect on the 5-HT-mediated facilitation of either the initial EPSC or EPSCs elicited late in the burst.