The development of obesity may be aggravated if obesity itself insulates against heat loss and thus diminishes the amount of food burnt for body temperature control. This would be particularly important under normal laboratory conditions where mice experience a chronic cold stress (at Ϸ20°C). We used Scholander plots (energy expenditure plotted against ambient temperature) to examine the insulation (thermal conductance) of mice, defined as the inverse of the slope of the Scholander curve at subthermoneutral temperatures. We verified the method by demonstrating that shaved mice possessed only half the insulation of nonshaved mice. We examined a series of obesity models [mice fed high-fat diets and kept at different temperatures, classical diet-induced obese mice, ob/ob mice, and obesity-prone (C57BL/6) vs. obesityresistant (129S) mice]. We found that neither acclimation temperature nor any kind or degree of obesity affected the thermal insulation of the mice when analyzed at the whole mouse level or as energy expenditure per lean weight. Calculation per body weight erroneously implied increased insulation in obese mice. We conclude that, in contrast to what would be expected, obesity of any kind does not increase thermal insulation in mice, and therefore, it does not in itself aggravate the development of obesity. It may be discussed as to what degree of effect excess adipose tissue has on insulation in humans and especially whether significant metabolic effects are associated with insulation in humans. obesity; insulation; ob/ob DESPITE THE PRESENT INTEREST IN METABOLISM in connection with the global obesity epidemic, there is little knowledge concerning the extent to which obesity as such affects metabolism. There is widespread interest in issues such as the release of adipokines from adipose tissue depots, but many basic issues, such as the ability of adipose tissue to affect metabolism physically, have been little examined. One of these issues is the question of the ability of adipose tissue to function as a thermal insulation barrier, decreasing the heat loss from the organism and in this way decreasing the amount of energy needed to keep the organism warm, thereby increasing the amount of excess energy prone to storage in the form of (additional) fat.Whether an insulating effect of obesity exists is of significance both for humans and for animal models of obesity. However, with regard to experimental animals, the issue of insulation is of further interest. This is because most metabolic experiments are conducted with mice kept under conditions (standard laboratory conditions of Ϸ21°C) that are principally cold for the mouse (10,19,27,37,38,42,44) and thus where a high proportion of total metabolism (nearly half) is devoted to counteracting the resulting heat loss. This is a condition very different from that which is relevant for the metabolic physiology of most humans (living in a thermoneutral environment). Thus, under the current experimental conditions for mice, an insulating effect of increasing obesit...