Using the indirect immunofluorescence method, the distribution of the Delta Sleep Inducing Peptide (DSIP)-containing neurons was studied in the rabbit brain. DSIP antisera were raised in rat by multiple injections of synthetic DSIP conjugated to thyroglobulin. Some DSIP immunoreactive cell bodies were detected in the diagonal band of Broca and anterior part of the hypothalamus. Large populations of immunofluorescent fibers and terminals were visualized mainly through the organum vasculosum of the lamina terminalis, the preoptic areas, the subfornical organ, the thalamus, the ventromedial hypothalamus and infundibulum. Further, most of the cells of the intermediate lobe of the hypophysis displayed DSIP-immunoreactivity. The predominant localization of DSIP-immunoreactive fibers and terminals in certain circumventricular organs suggests that DSIP could play a specific role in the neurohumoral regulation.