The cellular location of Neuropeptide Y (NPY) synthesis in rat brain is identified by using in-situ hybridisation histochemistry. The results show that NPY mRNA is widely distributed through the rat brain, although the levels of NPY mRNA are surprisingly low. There is a large degree of variation in the content of NPY mRNA in different regions. The highest cellular levels of NPY mRNA are found in the arcuate nucleus of the hypothalamus, in the hypothalamus, in the cerebral cortex, and in the hilar region of the hippocampus. In general, the distribution of perikarya containing NPY mRNA corresponds to the reported distribution of perikarya containing NPY-immunoreactivity. However, NPY mRNA was detected in the majority of the perikarya in the reticular nucleus of the thalamus, an area not previously known to contain NPY neurones. In many areas of the forebrain the distribution of NPY mRNA parallels that of somatostatin mRNA, supporting suggestions of their coexistence. The ability to detect NPY mRNA at the cellular level should be of considerable use in dynamic studies of the activity of NPY neurones.