In the present study, lidocaine, a local anesthetic that inhibits the initiation or conduction of nerve impulses, was used to differentiate between the memory for the conditioned stimulus (CS) and the memory for the CS and uncondi-tioned stimulus (US) association. Lidocaine was used to block memory forma-tion. It was administered into the cistema magna to localize the inhibition to the central nervous system (CNS) where circuits for the CS and US exist. The results show that lidocaine specifically blocks the ability of the CS to stimulate the circuits responsible for storing the CS/US association, but it does not inter-fere with the inherent ability of the US to signal the CNS and trigger a periph-eral response. The observation that the CS circuit can be interrupted indepen-dently of the US circuit suggests that these signals come together to form a new circuit for the memory of the association. The association memory forms later and is independent of the memory for the CS. © 1994 S. Karger AG, Basel.