To get more insight into developmental aspects of neuropeptide Y (NPY)-containing neuronal structures in the brain of amphibians and their possible involvement in background adaption, we have studied immunohistochemically the distribution of this neuropeptide in embryos, larvae and adults of Xenopus laevis. Antisera against NPY revealed that already at early embryonic stages NPY immunoreactive cell bodies are present in the ventral thalamus and rhombencephalic tegmentum. Slightly later, cell bodies appear in the olfactory bulb, the basal forebrain including the lateral and medial amygdala, the preoptic area, the ventral and dorsal thalamus, the suprachiasmatic region, the anteroventral tegmental nucleus and the solitary tract area. At late embryonic stages, the NPY cell groups not only show an increase in number of cells, but also stain more intensely. Around the time of hatching, a dramatic decrease in the number of immunodetectable cells occurs, particularly in the basal forebrain and in the rhombencephalic tegmentum. At the same time, however, new cell groups appear in telencephalic pallial regions and in the torus semicircularis. By the end of the premetamorphic stages, the distribution of NPY-immunoreactive cell bodies and fibers resembles closely the pattern observed in adult Xenopus brains. When compared with the development of catecholamine systems, it is clear that the NPY neurotransmitter system develops earlier. However, the expression of NPY- and dopamine-immunoreactivity in the suprachiasmatic nucleus occurs at about the same time (around stage 40) and coincides with several other events related to background adaptation, suggesting that this nucleus plays a key role in this complex neuroendocrine mechanism.