The auditory response of locus coeruleus (LC) neurones evoked by novel tones was investigated in anaesthetized and awake rats. Recording the single unit activity of LC neurones, responses to auditory stimuli are found under anaesthesia as well as in the awake animal. There are three types of LC responses to tone: first and by far the most frequent, a burst of several spikes at onset of the tone; second, a burst at tone offset and lastly, a total inhibition to the tone. All responses present a rapid habituation after the first few presentations of the stimulus. The results support the view that the LC plays a role in mediating responses to environmental changes.