The ultrastructural appearance of the nuc. caudatus putamen (NCP) of the rat was studied after unilateral lesions of the nigro‐neostriatal dopamine (DA) fibres. A strong reduction in the percentage of boutons containing small granular vesicles was found in slices of the NCP on the operated side after incubation in an α‐methyl‐noradrenaline (NA) (10 Mg/ml) containing medium. This reduction was of about the same magnitude in rats killed two to twelve days after the operation, i.e. from about 12 per cent on the control side to 0–4 per cent on the operated side. However, no certain reduction was found in animals killed one day after the operation. These results were in perfect agreement with the fluorescence microscopical findings, demonstrating a reduced accumulation of amines in the DA nerve terminals as well as reduced levels of endogenous amine at these time intervals after the lesion.–Parallel to the reduction in number of boutons containing small granular vesicles, boutons were observed showing obvious signs of the so called “dense” type of degeneration. These boutons were, however, comparatively rare and present above all in rats killed between two and four days after performing the lesion. Further studies are necessary to establish whether they represent boutons belonging to degenerating DA nerve terminals or not. It may be concluded that the present study gives ultrastructural evidence for the existence of an ascending nigro‐neostriatal DA neuron system, although the degenerating pattern of these neurons is still unclear. © 1969 Scandinavian Physiological Society
