Synaptic plasticity is thought to contribute to memory formation. Serotonin-induced facilitation of sensory-motor (SN-MN) synapses in "Aplysia" is an extensively studied cellular analog of memory for sensitization. Serotonin, a modulatory neurotransmitter, is released in the CNS during sensitization training, and induces three temporally and mechanistically distinct phases of SN-MN synaptic facilitation. The role of protein kinase A and protein kinase C in SN-MN synaptic facilitation is well documented. Recently, it has become clear that mitogen-activated protein kinase (MAPK) cascades also play a critical role in SN-MN plasticity. Here, we summarize the roles of MAPK cascades in synaptic plasticity and memory for sensitization in "Aplysia." (Contains 1 figure.)