In Lymnaea aerial respiratory behaviour can be operantly conditioned; snails learn not to perform this behaviour. Depending on the training procedure used, snails are competent to form either intermediate-term (ITM; lasting 1-3 h) or long-term (LTM; >4 h) memory. We found that cooling the snails for 1 h immediately after training was sufficient to block either ITM or LTM. Cooling snails for a similar period 10 or 15 min after cessation of training, failed to block ITM and LTM formation, respectively. Finally, we employed the cooling technique to extend both ITM and LTM. That is, cooling could prevent forgetting. Cooling extended LTM that normally persisted for 2 days to at least 8 days. These data are consistent with the hypothesis that forgetting is due to the learning and remembering of interfering events, and thus is an active process.