The distribution of vasoactive intestinal peptide (VIP) binding sites in the brain of several vertebrate species was examined by in vitro autoradiography on slide-mounted sections. This study included fish, frog, snake, pigeon, rat, mouse, guinea pig, cat and monkey brain. A fully characterized, monoiodinated form of vasoactive intestinal peptide (M-125I-VIP), which maintains the biological activity of the native peptide in the central nervous system (CNS), was used throughout the study. Among the lower vertebrate species, no significant specific binding was found in the fish brain, whereas in the frog and snake brain, specific VIP binding sites were observed, mainly in the telencephalon. In the pigeon brain, high densities of VIP binding sites were localized in the hyperstriatum, neostriatum, archistriatum, hippocampal area, dorsolateral cortical area and in the optic tectum. Ectostriatum and paleostriatum augmentatum displayed lower densities of specific binding. In mammals, the highest concentrations of VIP binding sites were observed in the rodent brain. In the rat, mouse and guinea pig brain, high densities were detected in the olfactory bulb, external layers of the cerebral cortex, dentate gyrus, midline thalamic nuclei, geniculate nuclei, some hypothalamic nuclei, superior colliculus and locus coeruleus. Intermediate densities were found in amygdala, caudate-putamen, septum and nucleus accumbens, CA1-CA3 fields of the hippocampus and central gray. The cerebellum of these species presented high densities of VIP binding sites, with species to species differences in their localization. The non-specific binding was, however, increased in the rodent cerebellum. Lower densities of VIP binding sites were observed in the cat and monkey CNS. In these two species, the non-specific binding was considerably higher than in the lower mammals brain. In the cat and monkey brain, as in the lower mammals, the highest densities were revealed in the neocortex, dentate gyrus, thalamic nuclei and some midbrain structures including substantia nigra and locus coeruleus. In all the species studied, the white matter was never labeled with M-125I-VIP. This study suggests that VIP binding sites appear relatively early in the evolution of the vertebrate CNS. The most important densities of specific VIP binding sites are observed in the pigeon and rodent brain, whereas the cat and monkey present a marked increase in non-specific binding. It is interesting to note that the distribution of VIP binding sites as revealed by autoradiography is quite conservative in terms of evolution and indicates an association, although non-exclusive, of VIP receptors with brain regions involved in the processing of specific sensory inputs.