The neuropeptide urotensin II (UII) is expressed in motoneurons of the brainstem and spinal cord in adults. Here, the expression pattern of the UII gene was studied in the developing rat spinal cord. UII mRNA was detected by reverse-transcription-polymerase chain reaction (RT-PCR) as early as E10. From E14 to E21, in situ hybridization revealed intense expression of the UII gene specifically in sacral motoneurons, while only faint expression was detected at cervical and thoracic levels. After birth (P0, P4), the expression of UII mRNA increased in motoneurons at all rostrocaudal levels. Thus, UII is the first gene reported to show expression limited to the sacral pool of motoneurons, which are known to have particular properties in terms of targets and programmed cell death.