ABSTRACT. Basal metabolic rate (BMR) represents the lowest level of metabolic activity capable to sustain homeostasis in an endotherm and is an important tool to compare metabolic rates of different species. Echimyidae is the most specious family within caviomorph rodents, however, little is known about the biology of its species, such as Trinomys setosus (Desmarest, 1817) and Clyomys bishopi (Ávila-Pires & Wutke, 1981), a ground and an underground dwelling echimyid, respectively. The ambient temperature and circadian effects on metabolic rate were evaluated through closed-system respirometry for these two species, as well as the circadian effects on CO 2 production and respiratory exchange ratio (RER). Trinomys setosus and C. bishopi showed the lowest metabolic rates (0.56 ± 0.02 mLO 2 .h , respectively) at 32 °C and during the light phase. Under laboratory conditions, T. setosus showed metabolic rate variation compatible with nocturnal activity, whereas C. bishopi activity cycle remains unclear. Both species showed BMR lower than expected by allometric regressions for rodents.