Serotonin-immunoreactive structures in the circumventricular organs (organum vasculosum laminae terminalis, subfornical organ, subcommisural organ and area postrema) of the rat were demonstrated using a modified peroxidase-antiperoxidase immunohistochemical method. Various densities of serotonin fibers were demonstrated in all four circumventricular organs; however, serotonin-positive cells were evident in the area postrema only after nialamide treatment. Serotonergic supraependymal fibers were observed on the surface of the organum vasculosum laminae terminalis and that of the subfornical organ, but not on the subcommissural organ and area postrema. The organ, but not on the subcommissural organ and area postrema. The serotonergic plexus of the basal portion of the subcommisural organ was considered to be continuous with the supraependymal plexus.