The efferent projections of the suprachiasmatic nucleus of the hypothalamus (SCN) have been investigated by the use of the anterograde autoradiographic technique. The majority of the fiber connections from SCN are to hypothalamic nuclei. The dorsal and periventricular parvocellular divisions of the paraventricular nucleus (PVN) and the medial part of the dorsomedial hypothalamic nucleus receive dense fiber projections. Also, fibers about the medial parvocellular division of PVN. Many descending fibers course into the retrochiasmatic area and pass to the ventral and medial borders of the ventromedial hypothalamic nucleus, as well as through the central zone of this nucleus. A few fibers at the lateral border of the ventromedial hypothalamic nucleus sweep dorsomedially into the posterior hypothalamic area and midbrain central gray. The major extrahypothalamic projection is to the periventricular thalamic nucleus.