Forgetting may allow an animal to react more appropriately to current conditions, rather than continuing to exhibit a previously learned, possibly maladaptive behaviour based on previous experience. One theory is that forgetting is an active process, whereby the previously learnt response is replaced by new learning that interferes with the older memory. Hence, we hypothesized that an appropriately timed environmental stressor that blocks long-term memory (LTM) formation would also block forgetting. Lymnaea stagnalis (L.) is a freshwater snail, which requires environmental calcium of at least 20 mg l(-1) to meet its requirements. Low environmental Ca(2+) (i.e. 20 mg l(-1)) in their environment acts as a stressor, and prevents LTM formation. Here, we asked whether a low Ca(2+) environment would also prevent forgetting, concordant with the retrograde interference model of Jenkins and Dallenbach. Snails were operantly conditioned to reduce aerial respiration in hypoxia. When maintained in standard conditions (80 mg l(-1) Ca(2+)), snails demonstrated LTM following training lasting 24 h, but not 72 h; however, when trained in standard conditions then exposed to a low Ca(2+) environment (20 mg l(-1)) immediately following training, they retained memory for at least 96 h, indicating that forgetting had been blocked. Thus, when exposed to low environmental Ca(2+), Lymnaea will fail to form new memories, but will also continue to retain information previously learned and remembered as the low calcium blocks forgetting.