Knowing the trophic ecology of the species that occur in a given location is crucial for understanding the factors that allow their co-existence. Using stable isotope ratios (δ¹³C and δ¹⁵N) and metrics of the isotopic niche, we evaluated the trophic level of 22 small mammal species from the Brazilian Cerrado (neotropical savanna) through the assimilation of three groups of food items (C₃ plants, C₄ grasses, and invertebrates). In addition, we investigated trophic assimilation during dry and rainy seasons in the three main Cerrado vegetation formations (grassland, savanna, and gallery forest). We assessed the effects of differences in availability and diversity of food resources and of variation in habitat complexity (i.e., increase in forest cover) on trophic behavior. Overall, we confirmed that omnivory is the predominant feeding category of small terrestrial mammals from the Brazilian savanna, but certain trophic specializations were detected, such as the high frugivory (C₃ source) of arboreal forest species (R. macrurus and O. cleberi) and insectivory of terrestrial swamp rats (Oxymycterus spp.). The amplitude of the trophic niche of some species increased as a positive response to food availability, both in forest and savanna areas in comparison to grassland areas, and in the rainy season compared to dry season. Inside forests, insectivorous rodents showed a broader isotopic niche during the rainy season (when resource availability is higher) than in the dry season but an opposite pattern arose for frugivorous species. Our results indicated that the influence of habitat complexity and food resources on the isotopic trophic niche is species-dependent and not unidirectional, thus being difficult to make general predictions of species response. Moreover, the patterns of isotopic assimilation indicated that habitat-generalist species are also isotopic generalists.