Objective: Assessment of the effect of a lowered ambient temperature, ie 16 C (61 F), compared to 22 C (72 F), on energy intake (EI), energy expenditure (EE) and respiratory quotient (RQ) in men. Design: Randomized within-subject design in which subjects stayed in a respiration chamber three times for 60 h each, once at 22 C, and twice at 16 C, wearing standardized clothing, executing a standardized daily activities protocol, and were fed in energy balance (EBI): no significant difference between EE and EI over 24 h). During the last 24 h at 22 C, and once during the last 24 h at 16 C, they were fed ad libitum. Subjects: Nine dietary unrestrained male subjects (ages 24 AE 5 y, body mass index (BMI) 22.7 AE 2.1 kg=m 2 , body weight 76.2 AE 9.4 kg, height 1.83 AE 0.06 m, 18 AE 5% body fat). Results: At 16 C (EB), EE (total 24 h EE) was increased to 12.9 AE 2.0 MJ=day as compared to 12.2 AE 2.2 MJ=day at 22 C (P < 0.01). The increase was due to increases in sleeping metabolic rate (SMR; the lowest EE during three consecutive hours with hardly any movements as indicated by radar): 7.6 AE 0.7 vs 7.2 AE 0.7 MJ=day (P < 0.05) and diet-induced thermogenesis (DIT; EE-SMR, when activity induced energy expenditure as indicated by radar ¼ 0): 1.7 AE 0.4 vs 1.0 AE 0.4 MJ=day (P < 0.01). Physical activity level (PAL; EE=SMR) was 1.63 -1.68. At 16 C compared to at 22 C, rectal, proximal and distal skin temperatures had decreased (P < 0.01). RQ was not different between the two ambient temperature situations. During ad libitum feeding, subjects overate by 32 AE 12% (at 22 C) and by 34 AE 14% (at 16 C). Under these circumstances, the decrease of rectal temperature at 16 C was attenuated, and inversely related to percentage overeating (r 2 ¼ 0.7; P < 0.01). Conclusion:We conclude that at 16 C, compared to 22 C, energy metabolism was increased, due to increases in SMR and DIT. Overeating under ad libitum circumstances at 16 C attenuated the decrease in rectal core body temperature.