The organization of retinohypothalamic tract (RHT) projections in the rat and hamster was studied using anterograde transport of cholera toxin conjugated to HRP (CT-HRP). In both species the major RHT projections lead to the suprachiasmatic nuclei (SCN). This projection begins in the rostral SCN as a loose plexus in the hamster and a a dense aggregation of terminals along the chiasmal border in the rat. Through the remainder of the SCN there is a very dense terminal plexus in the ventral and lateral part of the nucleus with fewer terminals present medially. The RHT projection to the SCN is greater contralaterally in the rat whereas in the hamster the contralateral and ipsilateral projections are approximately equal. In addition to projections to the SCN, the RHT projects to the anterior hypothalamic area, the retrochiasmatic area and lateral hypothalamic area in both species. The anterior hypothalamic projections are more extensive in the hamster than in the rat and extend into the perifornical region, the dorsal hypothalamus and zona incerta. The SCN and anterior hypothalamic projections are continuous with a projection to the retrochiasmatic area and, in the hamster, with a projection extending into the subparaventricular zone with some axons and terminals continuing into the paraventricular nucleus. In contrast to these, the lateral hypothalamic projection in the rat is more extensive than in the hamster. Albino and pigmented rats show identical projections. In addition to the hypothalamic projections, there is in the hamster a small projection along the base of the telencephalon to the anterior amygdaloid area and cortical amygdaloid nucleus and a very sparse projection to the anterior thalamic nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)