Lymnaea stagnalis were operantly conditioned to not perform aerial respiratory behavior. This learned response was subsequently extinguished. Here, we show that spaced extinction training is more effective than massed extinction training, in addition to the occurrence of spontaneous recovery. We also find evidence of a critical period within the first hour after extinction training in which new RNA and protein synthesis must occur for a memory of extinction training to be established. The memory for extinction training can also be extended using cooling and by preventing aerial respiration from occurring after extinction training. In addition, we demonstrate that memory formation of extinction training requires the soma of the cell right pedal dorsal 1, a cell that we have previously shown to be necessary for long-term memory consolidation and reconsolidation. This finding implies that the events that lead to the formation of extinction memory occur in the same cell that is responsible for long-term memory of operant conditioning. All of these data are consistent with the hypothesis that, during extinction, a new associative memory is being formed and that this new memory covers up, but does not abolish, the "old" memory.