Physiological mechanisms causing reduction of metabolic rate during torpor in heterothermic endotherms are controversial. The original view that metabolic rate is reduced below the basal metabolic rate because the lowered body temperature reduces tissue metabolism has been challenged by a recent hypothesis which claims that metabolic rate during torpor is actively downregulated and is a function of the differential between body temperature and ambient temperature, rather than body temperature per se. In the present study, both the steady-state metabolic rate and body temperature of torpid stripe-faced dunnarts, Sminthopsis macroura (Dasyuridae: Marsupialia), showed two clearly different phases in response to change of air temperature. At air temperatures between 14 and 30 degrees C, metabolic rate and body temperature decreased with air temperature, and metabolic rate showed an exponential relationship with body temperature (r2 = 0.74). The Q10 for metabolic rate was between 2 and 3 over the body temperature range of 16 to 32 degrees C. The difference between body temperature and air temperature over this temperature range did not change significantly, and the metabolic rate was not related to the difference between body temperature and air temperature (P = 0.35). However, the apparent conductance decreased with air temperature. At air temperatures below 14 degrees C, metabolic rate increased linearly with the decrease of air temperature (r2 = 0.58) and body temperature was maintained above 16 degrees C, largely independent of air temperature. Over this air temperature range, metabolic rate was positively correlated with the difference between body temperature and air temperature (r2 = 0.61). Nevertheless, the Q10 for metabolic rate between normothermic and torpid thermoregulating animals at the same air temperature was also in the range of 2-3.(ABSTRACT TRUNCATED AT 250 WORDS)