Six days after bilateral experimental ablation of the rat retina the nuclear diameter of the suprachiasmatic neurons decreased by 10.3% (Wenisch und Hartwig 1973). From these results it has been postulated that a direct retinohypothalamic connection exists between the eye and the suprachiasmatic nuclei. When unilateral retinal ablation was performed, a decrease in nuclear diameter was only observed in the contralateral suprachiasmatic nucleus (11.3% decrease, similar to that in bilaterally blinded rats). In the ipsilateral nucleus, as was the case in the sham-operated animals, no change in nuclear diameter occured. The nuclear diameter of the supraoptic nucleus, the medial geniculate nucleus and the ipsilateral geniculate nucleus 6 days after unilateral retinal ablation did not show any change, as was the case for these same nuclei in the sham-operated rats. However in the contralateral lateral geniculate nucleus an increase of 10.9% in nuclear diameter was observed. This apparent difference in the observed response of the lateral geniculate nucleus, as compared to the suprachiasmatic nucleus, could possibly be related to the greater distance of this area from the retina; the degeneration of the suprachiasmatic nucleus having been initiated earlier. From the silver-impregnation methods of Nauta-Fink-Heimer and Bodian-Ziemer applied to demonstrate degenerating nerve fibers and terminals 10 days after unilateral and bilateral ablation of the retina no definite conclusions could be drawn for the existence of a direct retinohypothalamic pathway.