The present study investigates the presence of nitric oxide synthase (NOS) by the nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemical reaction and immunohistochemistry with neuronal NOS (nNOS) antibody during postnatal development of hypoglossal nucleus (XII) in the rat. Our results showed that the enzyme expression was limited to a subset of cells in the dorsal division of the nucleus. Retrogradely labeling with diamidino yellow (DY) from the tongue indicates that these cells were motoneurons. NOS expression was transient, being detectable during the first 3 postnatal weeks, suggesting a role for nitric oxide in the development and maturation of this subset of cranial motoneurons.