Histochemical studies have been conducted on the distribution of labeled neurons in the rat pons at the level of the locus coeruleus after HRP injections into the supraoptic nucleus. In all the fifteen cases in which the supraoptic nucleus was found to be included in the injection site, labeled neurons were detected in the locus coeruleus whereas in the other nine cases in which the injection site was outside the supraoptic nucleus, labeled neurons were not detected in the locus coeruleus. Six cases having the injection site successfully localized within the supraoptic nucleus showed definitely a few labeled multipolar neurons only in the ipsilateral locus coeruleus. These findings demonstrate the presence of an efferent projection arising from the locus coeruleus to the ipsilateral supraoptic nucleus. It is suggested that mainly multipolar neurons in both divisions of this nucleus may contribute to this projection. There remains a possibility that the locus coeruleus may project to the supraoptic nucleus on the contralateral side.