Through the use of animal models, specific forms of mentation can now be explored on the cellular and molecular levels. Chronic anxiety and anticipatory anxiety in humans are closely paralleled by two forms of learned fear in the sea snail Aplysia: sensitization and aversive classical conditioning. In Aplysia's simple nervous system it is possible to delineate how the two forms are acquired and maintained. Both rely on the mechanisms of presynaptic facilitation. An augmented form of presynaptic facilitation accounts for the associative component of conditioning. These findings suggest that a surprisingly simple set of cellular and molecular mechanisms in various combinations may underlie a wide range of both adaptive and maladaptive behavioral modifications.