Axonal growth is now commonly recognized as one of the basic mechanisms underlying neuronal plasticity and reorganization in the adult mammalian CNS, not least in response to traumatic lesions. The monoaminergic neurons-cate- cholamine (CA)1-containing and indolamine (IA)-containing ones-in the adult rat brain have previously been shown to possess a high capacity for axonal growth and sprouting, both from their proximal axon stumps after axonal transection (Katzman et al. , 1971; Bjorklund et al. , 1971 c) and from their intact axons after partial denervations of different terminal regions (Moore et al. , 1971; Stenevi et al. , 1972). It seems possible that the monoamine neurons could be representative of a class of neurons in the CNS that retain a high degree of morphological plasticity also in the fully developed animal. This study was therefore undertaken to further characterize the growth properties, and particularly the regenerative capacity, of the central noradrenaline (NA), dopamine (DA) and IA neurons after axonal transection, using transplantations of the iris to various central locations as an experimental model. In previous studies (Bjorklund and Stenevi, 1971; Bjorklund et al. , 1971 c; Stenevi et al. , 1974), the regrowing sprouts from lesioned central NA and DA neurons have been shown to grow in great abundance into transplants of peri- pheral tissue placed in the caudal diencephalon or the spinal cord.