We examined the effects of caloric stimulation on the neuronal activity of the locus coeruleus (LC) and of the vestibular nucleus complex (VNC) in urethan-anesthetized rats. The single unit activity of neurons in the LC and VNC was extracellularly recorded. A polyethylene tube for caloric stimulation was inserted into the middle ear cavity on the ipsilateral side. Through the tube, the middle ear was irrigated by hot (44 degrees C), cold (30 degrees C), and ice (4 degrees C) water. The majority of neurons in the VNC showed excitation by middle ear irrigation with hot water and inhibition by ice-water irrigation. The responses occurred during caloric stimulation and disappeared immediately after the cessation of the stimulation. The results suggest that the responses of VNC neurons to caloric stimulation directly reflect changes in primary vestibular afferent activity. On the other hand, the predominant effect of caloric stimulation with hot and cold water on LC neuronal activity was inhibitory. The suppression of LC neuronal activity occurred approximately 1 min after the cessation of the caloric stimulation and persisted for 3-5 min. The results suggest that LC neurons receive processed vestibular signals. Motion sickness and vestibular dysfunction induced by caloric stimulation cause emesis, which is known as vestibulo-autonomic response. The vestibulo-autonomic syndrome can be prevented by amphetamine, a noradrenaline releaser. Therefore, the inhibitory response of noradrenergic LC neurons to vestibular stimulation may be involved in the vestibulo-autonomic response.