Measurements were made of portal blood flow, heat production and oxygen consumption in the digestive tract of sheep either fasted or given the following diets: chopped, dried grass, pelleted, dried grass; chopped, dried lucerne; pelleted, dried lucerne, or a pelleted barley diet. 2. For sheep that had been fasted for 48 h, portal blood flow was 1.84 l/min, total visceral heat production was 62.3 kJ/kg body-weight 0.75 per 24 h and aerobic heat production, estimated from oxygen consumption, was 62.1 kJ/kg body-weight 0.75 per 24 h. 3. Portal blood flow was markedly influenced by food intake, increasing from 1.8 l/min for starved sheep to 2.4 and 4 l/min for sheep feed at maintenance and 2.5 times maintenance levels of intake respectively. Variations in the quality and physical form of the diets had no apparent effect on portal blood flow. 4. There was a curvilinear relationship between total heat production in the gut and metabolizable energy (ME) intake. The increase obtained for levels of intake below maintenance was greatest with lucerne diets, and least with pelleted, dried grass or pelleted barley diets. Above maintenance levels of intake the rate of increase in heat production, with all diets, was about 150 kJ/MJ ME intake. 5. The heat of fermentation, estimated from the difference between total visceral metabolism and the aerobic metabolism of the tissues of the gut wall, was 76, 60 and 22 kJ/MJ digestible energy intake for the dried grass, lucerne and barley diets respectively. 6. The contribution of fermentation heat and the aerobic metabolism of the gut to the total heat increment of feeding in sheep was assessed. It was concluded that about half the heat increment must be derived from tissues outside the digestive tract.
