[en] The primary sensory neurons innervating the upper and lower first molars of the rat were studied by ³H-HRP autoradiography with respect to their localization in the trigeminal ganglion. The results showed labelled neurons were found only in the ganglion ipsilateral to the injection; the lablled neurons innervating upper and lower first molars might be divided into somatotopic areas, however, they intermingled and overlapped in the ganglion. The present findings seem to be of value for studying the mechanisms of odontogenious referred pain