The wild white-footed mouse, Peromyscus leucopus, is commonly used for photoperiod studies utilizing physiological, behavioral, and other biological measures indicative of hypothalamic functions. Indoleamines, like melatonin and serotonin, are implicated in regulating these hypothalamic functions. Although neurochemical analyses of hypothalamic serotonin and its receptors have been reported for this species, the relevant neuroanatomy of the serotonin system within mouse hypothalamus has not been studied. A sensitive immunohistochemical method was used to detect serotonin within axons of coronal sections of formaldehyde fixed forebrain from P. leucopus. Large, medium and small diameter serotonin axons were evaluated in most regions, or nuclei, of the hypothalamus rostral to the mammillary region. A fourth type of serotonin axon was observed to have morphology characteristic of terminal arbors. The density of serotonin axons ranged from no staining to very high density similar to other species for which reports exist, i.e., rat, cat, and monkey. The ventromedial hypothalamic nucleus had distinctively lesser density of serotonin axons in this mouse than other species. Evidence of terminal arborization in hypothalamic nuclei and regions was evident. Neuroendocrine, autonomic, and behavioral functions of the hypothalamus are suggested to be regulated by input from serotonin terminals in this wild species of mouse, in correlation with receptor localization as reported by others.