Ambient temperature has been shown to affect energy metabolism in field situations. Therefore, we assessed the effect of a short exposure to the thermoneutral zone, i.e., 27 degrees C (81 degrees F), in comparison to the usual ambient temperature of 22 degrees C (72 degrees F), on energy expenditure (EE), substrate oxidation, and energy intake (EI) in a controlled situation. Subjects, i.e., women (ages 22+/-2 years, BMI 22+/-3, 28+/-4% body fat), stayed in a respiration chamber three times for 48 h each: once at 22 degrees C, and twice at 27 degrees C in random order, wearing standardized clothing, executing a standardized daily-activities protocol, and being fed in energy balance (EB). During the last 24 h at 22 degrees C, and once during the last 24 h at 27 degrees C, they were fed ad libitum. At 27 degrees C, compared to at 22 degrees C, EE was 8.9+/-1.3 MJ/day vs. 9.9+/-1.5 MJ/day (P<.001) due to decreases in diet-induced thermogenesis (DIT) and activity-induced energy expenditure (AEE) (P<.01); respiratory quotient (RQ) had increased (P<.05); core (P<.05) and skin (P<.001) temperatures had increased. During ad lib feeding, EI was 90-91% of EE (P=.9), due to changes in energy density (ED) of the food choice (P<.01), and related to changes in body temperature and EE (P<.001). Thus, at 27 degrees C, compared to 22 degrees C, energy metabolism was reduced by reductions in DIT and in AEE, while RQ was increased. Reduction in EI was primarily related to body temperature changes and secondarily to changes in EE.