Helox (79% helium and 21% oxygen) has often been used for thermobiological studies, primarily because helium is thought to be metabolically inert and to produce no adverse effects other than increasing heat loss. However, these assumptions have been questioned. As basal metabolic rate (BMR) represents maintenance energy requirements for vital body functions, potential physiological effects of helox should be reflected in changes of BMR. In this study, sugar gliders were subjected to both air and helox atmospheres over a wide range of T(a)'s, including the thermoneutral zone (TNZ), to determine (1) whether helox has any influence other than on heat loss and (2) the maximum heat production (HP(max)) and thermal limits of this species. Although thermal conductance in the TNZ increased in helox, BMR was similar in air and helox (0.55+/-0.07 and 0.57+/-0.06 mL g(-1) h(-1), respectively). The TNZ in helox, however, was shifted upwards by about 3 degrees C. Below the TNZ, sugar gliders were able to withstand an effective temperature of -24.7+/-7.3 degrees C with an HP(max) of 3.14+/-0.36 mL g(-1) h(-1). The low effective temperature tolerated by sugar gliders shows that they are competent thermoregulators despite their apparent lack of functional brown fat. Similarities of BMRs in air and helox suggest that the effect of helox is restricted to an increase of heat loss, and, consequently, helox represents a useful tool for thermal physiologists. Moreover, the lack of increase of BMR in helox despite an increase in thermal conductance of sugar gliders suggests that BMR is not a function of body surface.