By means of quantitative receptor autoradiography using 125I-neuropeptide Y (125I-NPY) as a radioligand, the distribution of 125I-NPY binding sites has been evaluated in coronal sections at various rostrocaudal levels of the medulla oblongata of the male rat. High densities of neuropeptide Y (NPY) binding sites were demonstrated in the nucleus tractus solitarius (nTS), in the nucleus paratrigeminalis, in the area postrema, in the medial nuclei of the inferior olive and in the substantia gelatinosa of the caudal part of the spinal trigeminal nucleus. Low densities were present in the dorsal motor nucleus of the vagus (dmnX) and in the hypoglossal nucleus. Other regions of the medulla oblongata showed only a very low density or no specific binding of 125I-NPY. These results indicate that the central cardiovascular actions of NPY at least in part may be mediated via an action in the nTS, in this way controlling the baroreceptor reflex activity. Neuropeptide Y mechanisms may also play a role in the regulation of other visceral afferents such as those involved in gastrointestinal control (dmnX) and of cerebellar function (inferior olive). Finally, the results indicate that a high density of NPY immunoreactive terminals in some regions of the medulla oblongata is associated with a low density of high affinity 125I-neuropeptide Y binding sites and vice versa.