Type 1 cannabinoid receptor (CB1) is the principal receptor for endocannabinoids in the brain; it mainly occurs in preterminal/terminal axons and mediates retrograde neuronal signaling mechanisms. A large body of physiological and electrophysiological evidence indicates the critical role of CB1 in the regulation of hypothalamic functions. Conversely, the distribution of CB1‐containing axons in the hypothalamus is essentially unknown. Therefore, we have analyzed the distribution and the ultrastructural characteristics of the CB1‐immunoreactive (IR) axons in the mouse hypothalamus by using an antiserum against the C‐terminal 31 amino acids of the mouse CB1. We found that CB1‐IR axons innervated densely the majority of hypothalamic nuclei, except for the suprachiasmatic and lateral mammillary nuclei, in which only scattered CB1‐IR fibers occurred. CB1‐IR innervation of the arcuate, ventromedial, dorsomedial, and paraventricular nuclei and the external zone of the median eminence corroborated the important role of CB1 in the regulation of energy homeostasis and neuroendocrine functions. Ultrastructural studies to characterize the phenotype of CB1‐IR fibers established that most CB1 immunoreactivity appeared in the preterminal and terminal portions of axons. The CB1‐IR boutons formed axospinous, axodendritic, and axosomatic synapses. Analysis of labeled synapses in the paraventricular and arcuate nuclei detected approximately equal numbers of symmetric and asymmetric specializations. In conclusion, the study revealed the dense and differential CB1‐IR innervation of most hypothalamic nuclei and the median eminence of the mouse brain. At the ultrastructural level, CB1‐IR axons established communication with hypothalamic neurons via symmetric and asymmetric synapses indicating the occurrence of retrograde signaling by endocannabinoids in hypothalamic neuronal networks. J. Comp. Neurol. 503:270–279, 2007. © 2007 Wiley‐Liss, Inc.