A small volume of radioactively labelled cadmium was injected into the tongue of rats. Two weeks later, the rats were killed and the lower brainstem with the hypoglossal nuclei was dissected out and sectioned in a cryostat. Autoradiography of freeze-dried sections showed accumulation of cadmium in both hypoglossal nuclei. When unilateral nerve section was performed prior to the injection, only the contralateral nucleus was labelled. The results are interpreted as strong evidence for retrograde axonal transport of cadmium in the hypoglossal nerve.