The objective was to characterize the ability of control and transgenic brown adipose tissue (BAT)-ablated uncoupling protein diphtheria toxin A chain (UCP-DTA) mice to adjust food intake in relation to changes in environmental temperature and to assess the involvement of leptin in this adjustment. We measured serum leptin in mice from a previous study of UCP-DTA mice raised at thermoneutrality (35°C) or at the usual rearing temperature (24°C) from weaning [Melnyk, A., M.-E. Harper, and J. Himms-Hagen. Am. J. Physiol. 272 (Regulatory Integrative Comp. Physiol. 41): R1088-R1093, 1997] and extended the study by acclimating control and obese UCP-DTA mice at 18 wk of age to cold (14°C) for up to 14 days. Leptin levels did not change in control mice at 14°C; however, food intake increased threefold within 1 day and remained at this level. Serum leptin level was elevated in UCP-DTA mice at 24°C compared with control mice at 24°C; this elevated level decreased within 1 day at 14°C and was not different from the level in control mice by 14 days. Food intake of UCP-DTA mice that were hyperphagic at 24°C did not change during 7 days at 14°C, then increased slowly. Similar low leptin levels were present in control mice raised at 24 or 35°C and in UCP-DTA mice raised at 35°C. Food intake of control mice raised at 24°C was two times that of control mice raised at 35°C. UCP-DTA mice raised at 35°C ate the same low amount as control mice raised at 35°C. UCP-DTA mice at 24°C were hyperphagic relative to control mice at 24°C yet had elevated leptin levels in their serum. Two principal conclusions are drawn. First, adjustment of food intake over a fourfold range by control mice acclimated to temperatures from 35 down to 14°C is independent of changes in serum leptin levels. Second, this adjustment of food intake in relation to temperature is defective in the UCP-DTA mouse; the defect leads to hyperphagia at 24°C and a failure to increase food intake as rapidly as control mice when exposed to 14°C. Because lack of UCP-1-mediated thermogenesis in BAT of knockout mice is known not to induce hyperphagia, we propose that deficiency of UCP-1-expressing brown adipocytes in BAT of UCP-DTA mice results in lack of a satiety factor, secreted by these cells in BAT of control mice in inverse relationship to sympathetic nervous system activity.