I compared the maximal aerobic metabolic rates (VO2max), field metabolic rates (FMR), aerobic reserves (VO2max-FMR), and basal metabolic rates (BMR) of wild and recently captured deer mice from low (440 m) and high (3800 m) altitudes. To separate the effects of the thermal environment from other altitudinal effects, I examined mice from different altitudes, but similar thermal environments (i.e., summer mice from high altitude and winter mice from low altitude). When the thermal environment was similar, VO2max, FMR, and aerobic reserve of low and high altitude mice did not differ, but BMR was significantly higher at high altitude. Thus, in the absence of thermal differences, altitude had only minor effects on the aerobic metabolism of wild or recently captured deer mice. At low altitude, there was significant seasonal variation in VO2max, FMR, and aerobic reserve, but not BMR. BMR was correlated with VO2max, but not with FMR. The significant positive correlation of BMR with VO2max indicates a cost of high VO2max, because higher BMR increases food requirements and energy use during periods of thermoneutral conditions.