The presence of brown adipose tissue (BAT) in marsupials is controversial because attempts to identify mitochondrial uncoupling protein (UCP) have been unsuccessful. Sminthopsis crassicaudata is a small nocturnal marsupial with an interscapular pad of adipose tissue. Electron microscopy revealed this tissue to have characteristics typical of BAT. GDP binding and UCP detection by immunoblot confirmed BAT. Expression of UCP was increased by cold exposure. When animals were placed from 28 to 15 degrees C, body temperature (Tb) decreased by 1.7 degrees C within 30 min and a further 1.0 degree C by 90 min (P < 0.001) before stabilizing at these lower levels. When animals were returned to 28 degrees C, Tb increased within 30 min (P < 0.001) and returned to basal by 120 min. When animals were maintained at 15 degrees C with ad libitum food for 12 days, Tb (P < 0.05), tail width (P < 0.04), and O2 consumption (P < 0.01) all decreased. The respiratory quotient increased (P < 0.001), indicating a change from fat to carbohydrate utilization. Food intake was unchanged, and body weight increased on day 1 (P < 0.01) before returning to baseline on day 3, remaining stable thereafter. These data suggest that although BAT is present in the marsupial S. crassicaudata, it may not be necessary for thermogenesis, at least in the short term. S. crassicaudata utilizes a plasticity in Tb and a change in substrate utilization to maintain energy balance and body composition without the need for an increase in metabolic rate or food consumption and without the need for torpor.