Activity-dependent facilitation is a mechanism of associative synaptic plasticity that contributes to classical conditioning in Aplysia. Previous studies of activity-dependent facilitation in the mechanosensory neurons of Aplysia suggested that the Ca2+ influx during paired spike activity enhances the transmitter-stimulated, cAMP- dependent, presynaptic facilitation in these cells. Moreover, paired activity was found to potentiate the activation of the adenylate cyclase by transmitter. It was therefore proposed that the Ca2+/calmodulin-sensitive cyclase may serve as a site of interaction between the inputs from the conditioned and unconditioned stimuli. These studies were carried out to test whether a Ca2+/calmodulin- sensitive adenylate cyclase in the Aplysia CNS has the properties necessary to mediate such an associative interaction. Three lines of evidence indicate that the same cyclase molecules that are sensitive to Ca2+/calmodulin are also stimulated by receptor to facilitatory transmitter via the stimulatory G-protein, Gs: First, calmodulin inhibitors reduced stimulation of the cyclase by facilitatory transmitter. When membranes had been preexposed to one of these inhibitors, trifluoperazine, the addition of exogenous calmodulin partially reversed the inhibition. Second, when Gs had been activated by GTP gamma S, so that it persistently activated the catalytic unit of the cyclase, stimulation of the cyclase by Ca2+ was greatly amplified, suggesting that the two inputs interact in activating a common population of the enzyme. Third, solubilized cyclase activity that bound to calmodulin-Sepharose in a Ca(2+)-dependent manner was stimulated by Gs, which had been partially purified from Aplysia CNS, as well as by Ca2+/calmodulin. Having demonstrated dual activation of the cyclase, we have explored the dependence of cyclase activation on the temporal pattern of Ca2+ and transmitter addition. Optimal activation required that a pulse of Ca2+ temporally overlap the addition of facilitatory transmitter. These several results suggested that the dually regulated adenylate cyclase might underlie the temporal requirements for effective classical conditioning in this system.