Extracellular recordings of the metacerebral cell (MCC), a serotonergic neuron in Aplysia, were obtained in free moving, undrugged animals. MCC activity was evoked by exposure to food. Arousal level was manipulated by satiating the animals or exposing them to a noxious stimulus. We found that the amount of evoked MCC activity correlated with the level of arousal of the animal.