A major difference between short- and long-term memory is that long-term memory is dependent on new protein synthesis. Long-term memory can be further subdivided into a transient, initial phase that is readily susceptible to disruption and a later, more stable and persistent stage. To analyze this transition on the cellular level, we have examined the steps whereby short-term facilitation is converted to a long-term form in the sensorimotor connection of the Aplysia gill-withdrawal reflex. We found that stable long-term facilitation (at 24 hr) requires a higher concentration (100 nM) of serotonin (5-HT) than does short-term facilitation (10 nM). By using low concentrations of 5-HT, which do not produce long-term facilitation, we now have been able to explore the intermediate phases between the short- and long-term processes. By this means we have uncovered a new transient phase that involves three mechanistically different mechanisms--covalent modification, translation, and transcription--each of which can be recruited as a function of the concentration of 5-HT.