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Precise knowledge of the ultrastructural features and interneuronal relationships of dopaminergic (DA) axon terminals or varicosities in neostriatum is still lacking. This ignorance is due to current limitations of the methods applicable to their specific visualization at electron microscopic level. High resolution radioautography, in particular, has not yet permitted a clearcut identification of the DA nerve endings which take up and store exogenous catecholamines in vivo, due to an apparent mobilization of tracer during standard histological preparative procedures for light and electron microscopy (Fig. 1 A). In this context, histological processing of the central nervous system by vascular perfusion, tested in adult rats subjected to prolonged lateroventricular instillation of [3H]DA, led to the following results and conclusions: 1 Axonal varicosities having accumulated [3H]DA in vivo may be detected in great number in the ipsilateral paraventricular neostriatum (Figs. 1 B and 1 C). 2 The specificity of this radioautographic labelling is evidenced by: (a) its disappearance or persistance, depending on the addition of a high concentration of non-radioactive noradrenaline or serotonin to the [3H]DA solution (Fig. 1 C); (b) the absence of labelled axonal varicosities in the supraependymal region (Figs. 2 E and 2 F) and suprachiasmatic nucleus (Fig. 1 E to be compared with 1 F) after administration of [3H]DA alone; (c) the absence of any localized accumulation of [3H]DA in neostriatum following prior destruction of the nigro-striatal DA system by 6-hydroxydopamine (Fig. 1 D). 3 It seems that carrying out the double fixation with glutaraldehyde and osmium by vascular perfusion is the prerequisite for retaining in situ the [3H]DA accumulated in vivo by neostriatal nerve endings. 4 Preliminary ultrastructural examination shows that the DA axonal varicosities of paraventricular neostriatum are of small caliber (mean diameter: 0.5 micron) and mostly contain clear synaptic vesicles, occasionally associated with a few larger dense-core vesicles (figs. 2 A-D). Several of these nerve endings establish axo-dendritic synaptic junctions (Fig. 2 C) and a few, perhaps, axo-somatic contacts (Fig. 2 D).