The location of cell somata of the motoneurons supplying the rat geniohyoid and thyrohyoid muscles and the peripheral course of their axons were studied by intramuscular injection of horseradish peroxidase in combination with severing the innervating nerves of these muscles. Labeled geniohyoid motoneurons were found ipsilaterally in the ventrolateral subnucleus of the hypoglossal nucleus, which separates at midlevel of this nucleus from the main hypoglossal nucleus lying near the central canal and finally projects to the dorsomedial part of the ventral horn in the transition level of the brain/spinal cord. Geniohyoid motoneuron axons mostly pass through the hypoglossal nerve, whereas axons of the motoneurons in the transition level, though much fewer in number, pass through the first cervical nerve. Labeled thyrohyoid motoneurons were found ipsilaterally to form a cell column that extends from the caudal end of the main hypoglossal nucleus in the transition level of the brain/spinal cord to the dorsomedial part of the ventral horn in the caudal first cervical segment. All their axons passed through the first cervical nerve, and thus, the rostral portion of the thyrohyoid motoneuron column, which appears to be part of the hypoglossal nucleus, is not included in this nucleus but constitutes the supraspinal nucleus, which connects the hypoglossal nucleus to the ventral horn motoneuron group. The mean cell body sizes of the geniohyoid and thyrohyoid motoneurons were 28.7 +/- 4.4 and 29.3 +/- 4.6 micrometers, respectively, and their cell body size distributions were unimodal.