In Lymnaea, aerial respiration can be operantly conditioned and depending on the training procedure employed two forms of memory can result: intermediate-term (ITM) and long-term memory (LTM). ITM, which persists for 3h, is dependent on de novo protein synthesis whilst LTM, which persists for at least 24 h, is dependent on both de novo protein synthesis and altered gene activity. A single 0.5 h training session (i.e. ITM-training) leaves behind a residual molecular memory trace, which a second bout of ITM-training can activate and boost it to a LTM. Here we extend this finding to show that either inhibiting protein phosphatase activity with okadaic acid (1 microM), or increasing protein kinase C (PKC) activity and therefore protein phosphorylation with bryostatin (0.25 ng/mL) treatment prior to ITM-training, results in a LTM. However, following right pedal dorsal 1 (RPeD1) soma ablation neither of these treatments are effective in producing LTM following ITM-training, indicating transcription is a necessity. These findings suggest that the balance between phosphorylation and dephosphorylation in neurons is a key factor for LTM formation.