To see if repeated mild stress causes plastic changes in central noradrenergic terminal axons, the density of terminal axons arising in locus coeruleus (LC) neurons of rats was quantified by antidromic stimulation technique. After the termination of stress treatments (immersion in warm water for 10 min daily) for 1 or 2 weeks, electrophysiological experiments were performed under urethane anesthesia. The frequency of LC neurons activated antidromically from the cerebral cortex increased in rats stressed for 2 weeks but not for 1 week. Since the increased frequency of antidromic responses was not due to a change in terminal excitability, the change observed here is considered to be morphological (terminal sprouting) rather than a physiological consequence. The results suggest that LC neurons dynamically alter their terminal morphology in response to environmental stimuli.