The location of efferent neurons in the brain stem of newborn kittens supplying the facial muscles was studied by means of horseradish peroxidase (HRP) used as a tracer of retrograde protein transport. Three to 15 microliters containing 0.3 to 1.5 mg of HRP was injected into the orbicularis oris and orbicularis oculi muscles of six kittens and in the seventh kitten, the ventral ramus of the facial nerve was transected and HRP was instilled on the cut end of the nerve. After a survival time of 24 hours, the animals were anesthetized, perfused by intracardic perfusion technique, and then they were fixed and the brain stems sectioned. Stains for peroxidase demonstrated the labelled neurons in the lateral division of facial nuclei in the ipsilateral side. The orbicularis oculi muscles were represented in the rostral pole of the lateral division of the facial nucleus dorsally while the orbicularis oris muscles were represented in the caudal pole of the lateral division of the facial nucleus ventrally. No labelled neurons were found in the nuclei of the Vth or any other brain stem nuclei. According to my knowledge, this method of localizing the neurons supplying the facial muscles has not been reported previously.