Abstract: Two experiments were conducted to determine the effects of fasting duration and body weight on the fasting heat production of Duroc x Large White x Landrace crossbred barrows. In exp. 1, 6 pigs were fasted for 96 h. Heat production was measured every 12 h and the experiment was repeated at 3 separate body weights (31.1±1.1, 50.3±0.8 and 89.7±1.2 kg) to investigate the effects of different fasting durations on heat production and substrate oxidation. In Exp. 2, 12 Duroc x Large White x Landrace crossbred barrows were used to determine the effect of different body weights on fasting heat production. Fasting heat production was measured at 35.6±2.4, 44.7±3.1, 53.4±2.8, 63.9±2.7, 73.4±2.4, 83.3±3.0 and 95.6±2.0 kg following a fast of 48 h for the 35.6 as well as 44.7 pigs and a fast of 72 h for pigs at the remaining body weights. Before fasting, pigs were fed twice daily at 09:00 and 17:00. Fasting heat production was measured using indirect calorimetry after pigs were deprived of feed and the temperature was controlled at 24±1°C for pigs weighing 35.6 and 44.7 kg and 22±1°C for pigs weighing 53.4-95.6 kg. The results of exp. 1 showed that the heat production decreased rapidly in the first 48 (31.1 kg) and 60 h (50.3 and 89.7 kg) of fasting (p<0.05) and then reached a stable period (p>0.05) between 48-96 (31.1 kg) and 60-96 h (50.3 and 89.7 kg) of fasting, respectively. In Exp. 2, with an increase in body weights (35.6-95.6 kg) the fasting heat production of pigs went up from 1577-2683 kcal day-1. In conclusion, the fasting heat production was 220 kcal/kg BW0.55/day (R2 = 0.97, p = 0.01) in pigs with body weights from 35.6-95.6 kg.