Classical conditioning of the withdrawal reflex of the marine snail Aplysia californica can be used as an important model system for investigating the neurobiology of associative learning. It results when weak tactile stimulation of the snail's mantle shelf or siphon is repeatedly paired with strong electrical shocks to the animal's tail. This learned behavioral change is thought to be mediated by a presynaptic neuronal mechanism-activity-dependent presynaptic facilitation of the connections between sensory and motor neurons in the CNS of Aplysia. Recent evidence suggests, however, that another type of synaptic plasticity-Hebbian potentiation of the sensorimotor connections-might contribute to classical conditioning in Aplysia. Additional evidence indicates that this relatively simple form of learning is likely to be mediated by multiple neuronal mechanisms.