An investigation of the developmental expression of calretinin immunoreactivity and mRNA expression was carried out in the developing mouse diencephalon. Attention was focused on the thalamic eminence, which is a prominent structure previously described in the thalamus of the fetal mammalian diencephalon and adult lower vertebrates. Calretinin-positive staining was first observed in the thalamic eminence beginning at embryonic day 11. In situ hybridization histochemistry confirmed the presence of calretinin mRNA in the thalamic eminence. During subsequent embryonic development calretinin expression was very intense in neurons in the thalamic eminence though embryonic day 17, and thereafter, was increasingly difficult to distinguish. By postnatal day 0 the thalamic eminence was no longer discernable. Additional neurons within the murine diencephalon also expressed calretinin positive cell bodies and, or neuronal processes, including the stria medullaris, the habenular commissure and the paraventricular thalamic nucleus. It is possible that the thalamic eminence may form during development in order to act as an organizing center for the diencephalon.