The distribution of dopamine (DA)-containing fibers in the virtual absence of noradrenaline (NA)-containing ones has been mapped by aldehyde fluorescence histochemistry in rats subjected to a combined neurotoxin treatment (intracerebral 6-hydroxydopamine injections plus systemic injections of the selective NA neurotoxin DSP-4). This pretreatment left di- and telencephalic DA levels largely unaffected, but reduced the NA levels by at least 86-96%. The resulting DA:NA ratios suggested that the catecholamine-containing structures, demonstrable by fluorescence histochemistry in the di- and telencephalic regions, were predominantly the DA-containing ones. While the distribution of DA terminal systems in the neo- and allocortical regions conformed well to previous results, the combined neurotoxin treatment revealed new features of the distribution of DA fibers in the diencephalon. In addition to the previously described innervations of the tubero-hypophyseal system, the incerto-hypothalamic system, and the mesohabenular pathway, previously unknown innervations were revealed in the supraoptic, paraventricular and dorsomedial nuclei of the hypothalamus, and in the paraventricular nucleus of the thalamus. Apart from some scattered fibers in the periventricular and lateral hypothalamic areas and medical zona incerta, other diencephalic nuclei seemed to be devoid of any significant DA terminal networks. The dopaminergic nature of these innervations is supported by DA uptake experiments (evaluated by fluorescence histochemistry) as well as by independent biochemical and immunohistochemical evidence. It is suggested that the DA innervations of the hypothalamic neurosecretory nuclei originate in cell bodies of the diencephalic A11-A14 cell groups and that such intradiencephalic DA projections participate in the regulation of oxytocin and vasopressin release from the pituitary.