An endocranial cast of the Late Cretaceous (?Middle Campanian) zalambdalestid Barunlestes butleri from the Gobi Desert in Mongolia is described and figured. The brain that produced this endocast had large, roughly pear-shaped olfactory bulbs; relatively wide lissencephalic cerebral hemispheres, strongly diverging posteriorly; the midbrain consisting of one pair of large colliculi extensively exposed on the dorsal side; relatively short and wide cerebellum. The rhinal fissure cannot be discerned with any certainty but the expansion of cerebral hemispheres suggests that the neocortex was possibly developed in this and other Cretaceous eutherian mammals.

Key words: Paleoneurology, endocranial casts, Barunlestes, Eutheria, Mammalia, Cretaceous, Mongolia.