The question we address here is whether a mild increase in environmental temperature affects body temperature and energy expenditure, focusing on the individual variation in the relation between energy expenditure and body temperature. We studied eight normal weight healthy females, 48 h at an ambient temperature of 22 degrees C, and 48 h at 27 degrees C. Energy expenditure (EE) was measured in a respiration chamber. Subjects' skin temperature was measured continuously from 8:00 a.m. until 12:00 p.m.: forehead, infraclavicular zone, thigh, hand, and foot. Core temperature was determined tympanically. Body composition was determined by under water weighing. Exposure to 27 degrees C caused a significant increase in body temperature (both skin and core), a decrease in temperature gradients, and a decrease in energy expenditure. At 27 degrees C 24 h EE, adjusted for body composition, was significantly related to body tympanic temperature. The decrease in 24 h EE, at 27 degrees C ambient temperature, was significantly, negatively related to the increase in T(tym), indicating individual responses in adaptation to elevated ambient temperature. Changes in temperature gradient (comparing 27 degrees C with 22 degrees C) were negatively related to changes in EE. This shows that individuals differ in their response to an increase in environmental temperature regarding the relative contribution of insulative or metabolic adjustments.