The efferent connections of the suprachiasmatic nucleus of the hypothalamus have been studied in the rat by the injection of 3H-proline into the nucleus and the surrounding regions of the rostral hypothalamus, and by the injection of the enzyme marker, horseradish peroxidase, into the region of the ventromedial hypothalamic nucleus. After an injection of 3H-proline confined to the ventral portion of the suprachiasmatic nucleus, transported label can be followed, in the autoradiographs, dorsally and caudally in the periventricular area as far as the caudal end of the ventromedial nucleus, into the triangular area between this nucleus and the arcuate nucleus, and along the ventral aspect of the tuberal region, just lateral to the ventromedial nucleus. A small number of silver grains are also seen over the internal lamina of the median eminence. No label can be followed rostrally or immediately lateral to the nucleus. Comparable injections into adjoining regions of the hypothalamus (especially the anterior hypothalamic area, the medial preoptic area, and the retrochiasmatic region) show transported label over the same regions, but with a somewhat different pattern of grain distribution; in addition, the anterior hypothalamic area shows an extensive projection through the medial forebrain bundle to the mammillary and supramammillary nuclei, the midbrain tegmentum, and certain of the midline thalamic nuclei. Although it is difficult in our autoradiographs to distinguish between the course of the efferent fibers from the suprachiasmatic nucleus and the zones in which they terminate, our evidence favors a termination among the cells of the periventricular area, and upon dendrites of the cells in the ventromedial, dorsomedial and arcuate nuclei, which extend beyond the limits of the nuclei into the periventricular area and to the area beneath the ventromedial nucleus.