Urotensin-II (UII), a 12 amino acid peptide, was discovered in the teleost fish neurosecretory cells located in the caudal portion of the spinal cord and which project to a neurohemal gland called the urophysis. The distribution of UII and of its prepro-UII mRNA is not limited to fish and was found for example in the rat spinal cord. In view of the potential interest of obtaining transgenic mice, we have therefore characterized the distribution of mouse pro-UII mRNA and UII immunoreactivity, by in situ hybridization and immunohistochemistry, respectively, in the mouse spinal cord. A population of UII-like immunoreactive cell bodies was located in the ventral horn of the different segments. These cells displayed all the features of motoneurons, as confirmed by a double immunohistochemical labelling showing the co-occurrence of UII and vesicular acetylcholine transporter, and by electron microscope immunocytochemistry. Retrograde labelling of motoneurons innervating the bulbocavernosus penile muscle showed that some of them contained UII. In situ hybridization histochemistry revealed that pro-UII mRNA was located in some ventral horn neuronal perikarya. The pro-UII mRNA-containing cell bodies possessed the same motoneuron characteristics, confirming the results of the immunohistochemical studies and showing that the gene of mouse UII is expressed in a subpopulation of motoneurons in the spinal cord. Our results support the assumption that UII peptide characterized as endocrine in fish is also expressed within mammalian motoneurons.