We evaluated brain size evolution in a specialized subterranean mammal, the ctenomyid rodent genus Ctenomys (tuco-tuco) and compared it, within a phylogenetic framework, to other caviomorph rodents differing in body size and modes of life. Although brain size in Ctenomys falls below the regression line obtained for caviomorph rodents, some fossorial species within the sister family Octodontidae, which make use of both above and below ground habitats, have a relatively larger brain size. Ctenomys has experienced a relatively recent and ‘explosive’ cladogenesis which yielded ∼60 living species differing almost one order of magnitude in body size. Based on the evolutionary lag hypothesis between body versus brain size evolution, a negative correlation between encephalization quotient and body size would be expected for a group experiencing such a recent diversification in size. This is because a decrease of body size occurring during the emergence of a new species should produce a correspondingly greater encephalization and vice versa. Despite the fact that the allometric coefficient of brain weight versus body weight for 30 living species of Ctenomys was lower than that obtained for families within Caviomorpha, we failed to find any significant relationship between encephalization and body weight. Finally, the relationship between brain size and metabolism was assessed for those species of Ctenomys in which metabolic data are available. Brain size evolution in Ctenomys is discussed in regards to sensory capabilities and behavioral attributes associated with the occupation of a subterranean ecological niche.