Summary Unilateral stereotaxic injections of 0.5–10 μg 5,6-dihydroxytryptamine (5,6-DHT) into the medial forebrain bundle (MFB) and the nigrostriatal dopamine (DA) bundle of rats caused a considerable decrease of 5-hydroxytryptamine (5-HT) and DA, but no or only a slight reduction of noradrenaline (NA) in the homolateral telediencephalon. On the contralateral side and in the caudal parts of the brain, no relevant diminution of the amines occured. The uptake of 5-HT and DA into brain slices and synaptosomes of the homolateral but not the contralateral telediencephalic regions was reduced. Application of 5,6-DHT in the dorsal NA bundle did not markedly affect the NA content of the telediencephalon. It is concluded that on intracerebral application 5,6-DHT does not show specificity only for the 5-hydroxytryptaminergic neurons, but also affects the dopaminergic system.