In studying the classical conditioning of the siphon withdrawal reflex in Aplysia, we have identified a neuronal mechanism that plays an important role in this conditioning: activity-dependent presynaptic facilitation. This review describes our analysis of the cellular basis of this associative mechanism. During the conditioning of the withdrawal reflex, the unconditioned stimulus, a tail shock, produces presynaptic facilitation of synaptic transmission from the siphon sensory neurons in the conditioned stimulus pathway. The facilitation is enhanced if a sensory neuron has fired action potentials just prior to receiving facilitatory input, as occurs during training when the conditioned stimulus precedes the unconditioned stimulus. This activity-dependent enhancement of presynaptic facilitation provides a mechanism for the temporal specificity in conditioning of the reflex. Activity-dependent facilitation appears to involve the same cyclic AMP (cAMP)-dependent cascade that underlies presynaptic facilitation in these neurons in the absence of paired spike activity. Our evidence suggests that it is the transient elevation of intracellular Ca2+ that is responsible for the enhancement of the facilitation response by paired spike activity. Moreover, our preliminary results indicate that Ca2+/calmodulin is able to potentiate the activation of adenylate cyclase in Aplysia neurons by facilitatory transmitter. Thus, the dual activation of the calmodulin-dependent cyclase by Ca2+ and transmitter may give this enzyme an important associative role in learning. In the conclusion, the possible phylogenetic generality of this associative mechanism is discussed as well as its possible role in activity-dependent processes in neuronal development.