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When studying the origin of long ascending fibers from the reticular formation of the brain stem in the cat, Brodal and Rossi ('55) observed that following lesions at or above the mesencephalon some nerve cells in the nuclei of the raphe showed retrograde cellular changes. The present study was undertaken in order to investigate more closely this ascending projection from the raphe and, furthermore, to see if evidence of other efferent Connections from this group of nuclei could be discovered.Only scattered observations concerning the efferent fiber connections of the nuclei of the raphe can be found in the literature. In Golgi material Cajal ('09-'11) describes axons of cells of the raphe nucleus of the medulla, entering the white matter of the reticular formation and giving off longitudinally running ascending (and decending) branches. In experimental studies of the fiber connections of the reticular formation in which the fiber degeneration which follows lesions of this has been studied (for example Nauta and Kuypers, ' 5 8 ) , the nuclei of the raphe may sometimes have been involved in the lesions. However, on account of the smallness of these nuclei and because bypassing fibers may have been involved, it can scarcely by this approach be decided whether degenerating fibers observed in such experiments actually originate in the nuclei of the raphe.The only experimental method which can be expected to give information on this point is to search for the occurrence of retrograde cellular changes in the raphe nuclei following lesions of their efferent fibers. This procedure has been used in the present work, which has been directed more specifically to the study of three possible categories of efferent connections : 1, fibers ascending in the brain stem; 2, fibers descending to the spinal cord, and 3, fibers passing to the cerebellum. As a necessary basis for this investigation a mapping of the normal topography and cytoarchitecture of the nuclear complex of the raphe was first made. The results of this are published separately (Taber, Brodal and Walberg, '60). MATERIAL AND METHODSSince previous experience in our laboratory has shown that in general the retrograde cellular changes are more marked and appear earlier in very young animals than in adult ones, the modified Gudden method (Brodal, '39, '40) was employed. The age of the kittens at the time of operation varied between 6 and 10 days, and the periods of survival were in most cases 5 to 9 days. The animals were all operated upon under Nembutal anesthesia and with sterile precautions. In some kittens lesions of the upper brain stem or the cerebral hemispheres were made, usually by inserting a knife supratentorially, in others partial transverse lesions of the spinal cord were made by means of a knife following laminectomy. Various lesions of the cerebellum were made following a subtentorial craniotomy, usually by suction.The animals were sacrificed under Nembutal anesthesia by cutting the vessels in the neck. The brain stem, spinal cord and cerebellum were ...