Abstract Iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L), were placed in different portions of the rat lateral geniculate nucleus, and the tracer was localized by immunohistochemistry. The efferent projections of the lateral geniculate to limbic areas were analysed with special reference to projections of the lateral hypothalamic area. Only in animals, in which neurons in the intergeniculate leaflet were labeled were PHA-L-immunoreactive axons in the lateral hypothalamic area observed. The nerve fibers were mostly located in the ventral and anterior parts of the lateral hypothalamic area. This tracing study suggests that the optic system probably mediating information about the photo-period influences not only the suprachiasmatic nucleus, but also the lateral hypothalamic area.