The presence and distribution of immunoreactivity for nitric oxide synthase type I and a panel of regulatory neuropeptides was investigated in the vomeronasal organ (VNO) of mouse embryos. Results show that nitric oxide synthase type I is first expressed in putative extrinsic nerve fibers reaching areas of vascular development at embryonic day 16 and in the vomeronasal nerve at embryonic day 15. Immunoreactivity for vasoactive intestinal peptide appears around developing vessels of the VNO during embryonic day 18. No immunoreactivity for atrial natriuretic peptide, substance P and calcitonin gene-related peptide is present in the VNO. It is concluded that, in the mouse, nitric oxide synthesis is a precocious event in the development of peripheral and central neural vomeronasal structures, representing a very early step in the neurochemical maturation of the VNO.