BACKGROUND The range of core temperatures not triggering thermoregulatory responses ("interthreshold range") remains to be determined in humans. Although the rates at which perioperative core temperatures vary typically range from 0.5 to 2 degrees C/h, the thermoregulatory contribution of different core cooling rates also remains unknown. In addition, sweating in women is triggered at a slightly greater core temperature than in men. However, it is unknown whether the vasoconstriction and shivering thresholds are comparably greater in women, or if women tolerate a larger range of core temperatures without triggering thermoregulatory responses. Accordingly, the authors sought to (1) define the interthreshold range; (2) test the hypothesis that, at a constant skin temperature, the vasoconstriction and shivering thresholds are greater during rapid core cooling than during slowly induced hypothermia; and (3) compare the sweating, vasoconstriction, and shivering thresholds in men and women. METHODS Eight men and eight women participated. The men participated on 2 separate days; no anesthesia or sedatives were administered. On each day, they were cutaneously warmed until sweating was induced and then were cooled by a central venous infusion of cold fluid. The cooling rates were 0.7 +/- 0.1 degrees C/h on 1 day and 1.7 +/- 0.4 degrees C/h on the other, randomly ordered. Skin temperature was maintained near 36.7 degrees C throughout each trial. The women were studied only once, in the follicular phase of their menstrual cycles, at the greater cooling rate. RESULTS The interthreshold range was approximately 0.2 degrees C in both men and women, but all thermoregulatory response thresholds were approximately 0.3 degrees C higher in women. All thresholds were virtually identical during slow and fast core cooling. CONCLUSIONS Our findings confirm the existence of an interthreshold range and document that its magnitude is small. They also demonstrate that the interthreshold range does not differ in men and women, but that women thermoregulate at a significantly higher temperature than do men. Typical clinical rates of core cooling do not alter thermoregulatory responses.