An autoradiographic method was used to assess the distribution of binding sites for [3H]neurotensin (NT) in the forebrain and midbrain of the pigeon. Within the telencephalon the highest levels of NT binding sites were observed within the hyperstriatum ventrale (HV). Moderate to high levels of NT binding were observed within the archistriatum, neostriatum intermedium, and hyperstriatum accessorium. These telencephalic regions and HV are thought to be comparable to portions of mammalian neocortex. Lower levels of binding sites were observed within the striatal complex including the laterally situated paleostriatum augmentatum and medially situated lobus parolfactorius. The lowest levels of NT binding sites in the telencephalon were observed within the paleostriatum primitivum (PP, considered comparable to mammalian globus pallidus), ectostriatum (comparable to layer IV of mammalian extrastriate visual cortex), field "L" (comparable to layer IV of mammalian auditory cortex), hippocampus, septum, and preoptic area. Despite considerable regional variation, the overall level of NT binding throughout the pigeon telencephalon appears to be significantly higher than that reported for mammals, particularly within pallial areas. Within the brainstem, moderate levels of NT binding sites were observed in the lateral habenular nuclei, the ventral tegmental area of Tsai, nucleus tegmentipedunculopontinus, pars compacta (comparable to the mammalian substantia nigra, pars compacta), locus coeruleus, and the nucleus subcoeruleus dorsalis. The latter four cell groups contain numerous catecholaminergic neurons. Corresponding catecholaminergic cell groups in mammalian forms also contain high levels of NT receptors. As in mammals, lower levels of NT binding were observed in most diencephalic nuclei. Somewhat higher levels of NT binding were observed within the pretectal nuclei spiriformis lateralis and spiriformis medialis. Moderate levels of NT binding sites were observed within the retinal terminal layers of the tectum (i.e., layers 1-7). Immunohistochemical experiments (Reiner and Carraway; Brain Res. 341:365-371, '85; Reiner: ARVO Abstracts: p. 185, '86) localizing NT and a related hexapeptide, LANT6, have shown that LANT6 is present in retinal ganglion cells, in cells of the paleostriatum, and in the striatotegmental and striatopretectal fiber pathways of the pigeon. Thus some features of the NT binding observed here in pigeon brain including the existence of substantial NT binding sites in the brainstem catecholamine nuclei, pretectum (nucleus spiriformis lateralis), and optic tectum may reflect the existence, at least in part, of functional receptors for LANT6.(ABSTRACT TRUNCATED AT 400 WORDS)