The present study describes the development of the thalamic reticular nucleus in rats with the use of Nissl staining and antibodies to parvalbumin and pro-alpha-thyrotropin-releasing hormone (alpha TRH). Two major subdivisions of the reticular nucleus are apparent: 1) the main body, which is itself heterogeneous and lies for the most part between the fibres of the internal capsule and external medullary lamina, and 2) the perireticular nucleus, which lies lateral to the main body and medial to the globus pallidus. In the main body of the reticular nucleus of adults, most cells in all regions are immunoreactive to parvalbumin and alpha TRH. During development there are two waves of parvalbumin and alpha TRH expression. The first wave occurs between postnatal day (P) 0 and P10, and labelled cells are apparent in rostrolateral areas of the main body of the nucleus only. At P10, such cells are not apparent. From P7 to adult, there is a second wave of parvalbumin and alpha TRH expression: labelled cells emerge first in central, then in caudal, and finally in rostral areas of the nucleus. In adults, the perireticular nucleus is made up of a few small cells which are immunostained for parvalbumin and alpha TRH. These cells are more frequent in areas of the internal capsule adjacent to the ventral regions of the main body of the reticular nucleus, rostrodorsal to the entopeduncular nucleus. From E (embryonic day) 17 to about P10, the perireticular nucleus consists of a surprisingly large population of neurones, many of which are parvalbumin and alpha TRH immunoreactive. By about P10, as in adults, there are few perireticular cells.