This study is an analysis of the regulatory mechanisms of plasticity. The first part provides a short review of the role of DNA-binding transcription factors in possible regulatory pathways and their activity in the mechanisms of plasticity. Our own data obtained in studies of the molecular mechanisms underlying the formation of conditioned defensive reflexes in Helix are then presented. These studies show that formation of defensive types of plasticity in Helix is accompanied by serotonin-induced translocation of a protein with Rf 0.58 and increases in G-protein activity, protein kinase A activity, and expression of the c-fos gene. Transcription factors CRE and AP-1 probably have roles in the learning process. Gel shift assays demonstrated the existence of transcription factors of the CRE and AP-1 families in adult snails. In juvenile snails, which were unable to form defensive types of plasticity, the serotonin protein with Rf 0.58 (the learning "marker") was absent from and was not induced in the CNS. Gel shift assay results also showed that transcription factors of the AP-1 family were not present and were not induced by serotonin or the protein kinase A activator forskolin, though these snails had significant levels of CRE transcription factors. Serotonin and forskolin increased the DNA-binding activity of CRE in juvenile Helix. The lack of activity of transcription factors of the AP-1 family in juvenile snails may explain their inability to development sensitization and conditioned defensive reflexes.