In adult rats, neurons displaying histochemical staining for 'non-specific' cholinesterase (ChE) are found 3 distinct regions of the dorsal thalamus: the thalamic reuniens nucleus (Re), the anterior dorsal nucleus (AD), and a region that includes the lateral part of the central lateral nucleus (CL) and the ventral portion of the lateral dorsal nucleus (LD). Normal development of ChE-positive neurons was studied with cholinesterase histochemical techniques in postnatal infant rats. Although ChE staining of capillary endothelium is detectable shortly after birth, ChE staining of neurons first occurs at about postnatal day 5 (PND 5) with light staining of AD and CL-LD. At PND 7, staining in AD and CL-LD has increased in intensity and staining also is present in neurons of the anterior ventral (AV) and ventral anterior (VA) nuclei. ChE staining of neurons in Re first appears at PND 10. The number of neurons staining for ChE in each of these nuclei, and also the intensity of staining in individual neurons, appear to increase during the next several days until about PND 14. After PND 14, ChE staining intensity in neurons of AD, Re, and CL-LD appears to plateau and the pattern of staining continues into adulthood. In contrast, ChE staining of neurons in VA declines markedly and only a very few neurons in the dorsal part of VA remain ChE-positive after PND 21. ChE staining of neuropil in AV increases markedly, obscuring somatal staining in this nucleus. These results are discussed in regard to transient and continued expression of ChE activity in the dorsal thalamus and possible functional roles of ChE.