The present paper describes the distribution of three calcium-binding proteins (calbindin D28k, calretinin, and parvalbumin) in the mouse dorsal claustrum and endopiriform nucleus. The three calcium-binding proteins were distinctly expressed in structures of both the claustrum and the endopiriform nucleus. Calbindin was the calcium-binding protein showing the highest expression in the claustrum and the endopiriform nucleus. In contrast, calretinin-immunoreactive structures, particularly cell bodies, were very scarce in these regions. Both calbindin-immunoreactive and parvalbumin-immunoreactive neurons were more abundant in the claustrum than in the endopiriform nucleus, and more in rostral than in caudal levels. Nevertheless, calcium-binding protein immunoreactive neurons constitute a minority population of claustral neurons. The colocalization study of calbindin and parvalbumin immunoreactivities has demonstrated that both calcium-binding proteins are mostly expressed by separate claustral neurons in the mouse. On the other hand, our results on parvalbumin and calretinin immunoreactivity match a novel subdivision of the mouse claustrum mostly based on the pattern of cadherin expression [Neuroscience 106 (2001) 505]. In this sense, we propose that a specific zone of the dorsal claustrum with cell bodies that strongly express Rcad and cadherin-8 would be the selective target for parvalbumin-expressing fibers, and that they would be mostly avoided by calretinin-expressing axons.