The distribution and density of neuropeptide Y (NPY)-immunoreactive nerve fibres and cells were determined in the urinary bladder of the horse by using the peroxidase-antiperoxidase (PAP) immunohistochemical method. Numerous undulating NPY-immunoreactive nerve fibres were found throughout the vesical wall, sometimes forming nerve bundles which ramified repeatedly as they coursed through the connective tissue septa to give rise to smaller bundles or single fibres which projected into the muscle fascicles forming muscular nerve plexuses, mainly in the bladder base. In the submucosa of this region, NPY-immunoreactive fibres formed a rather dense subepithelial plexus. Numerous NPY-immunoreactive nerve fibres supplied blood vessels and were widely distributed on the vascular adventitia constituting rich perivascular nerve plexuses. In addition, intramural ganglia containing NPY-immunoreactive nerve cell bodies and fibres were identified at the uretero-vesical junction. These results suggest that the equine urinary bladder possesses a rich NPY-peptidergic innervation which shows regional variations in the density of the muscular and subepithelial plexuses, the bladder base being the most richly innervated region. At least some of these NPY-immunoreactive nerve fibres have an intrinsic origin in ganglion cells within the vesical wall.