In a number of neuronal models of learning signalling by endogenous nitric oxide (NO), produced by the enzyme NO synthase (NOS), is essential for the formation of long-term memory (LTM). For example, in the molluscan model system Lymnaea, NO is required for LTM formation in the first few hours after one-trial reward conditioning. Furthermore, conditioning leads to transient up-regulation of the NOS gene in identified modulatory neurons, the cerebral giant cells (CGCs), which are known to be involved in LTM formation. In Lymnaea nothing is known however about the structure and localization of the major receptor for NO, the soluble guanylyl cyclase (sGC). Here we report on the cloning and characterization of both α and β subunits of NO-sensitive sGC and show that they are coexpressed in the CGCs. Furthermore, our electrophysiological experiments on isolated CGCs show that these neurons respond to NO by generating a prolonged depolarization of the membrane potential. Moreover, we demonstrate that this depolarization is blocked by ODQ, supporting our hypothesis that it is mediated by sGC.