The postnatal growth of motoneuron cell bodies located in the brainstem, cervical and lumbosacral spinal cord was investigated using retrograde transport of horseradish peroxidase in kittens ages 2, 12, 30, 55, 82 and 114 postnatal days and in an adult. The motoneurons innervating an extrinsic tongue muscle, the genioglossus, reached their adult size by eight weeks after birth. In contrast, the phrenic motoneurons innervating the diaphragm achieved adult size by 12 weeks and the motoneurons innervating the medial gastrocnemius muscle continued to grow beyond the twelfth postnatal week. The sizes of these motoneurons relative to one another remained constant during periods of development.