1. A method is described for measurement of portal blood flow in the sheep by the principle of continuous thermal dilution produced by injection of cool saline into the portal circulation at 1 mg/s for 30 s.

2. In most animals phlebitis developed only around the catheter in the right ruminal vein. This affected their appetite and condition after surgery. When all catheters were introduced into the anterior mesenteric veins there was no phlebitis and appetite postoperatively was normal.

3. Portal blood flow in six out of nine sheep was 33–34 mg/kg body-weight per min in the morning before feeding. After a meal of dried grass it increased during the period about 2–6 h later.

4. Total heat production and oxygen consumption of the digestive tract were estimated from portal blood flow and arteriovenous differences in temperature and oxygen concentration. When the heat production of the digestive tract was increased by 1·77 kJ/min by the inclusion of a heating element in the rumen, estimated total heat production increased, on average, by 1·73 kJ/min.

5. During the period 2–6 h after a meal of dried grass, portal venous temperature rose, on average, by 0·7°, visceral O2 consumption did not alter significantly, but total heat production rose by 1·19 kJ/min.

6. These initial results suggest that the heat of fermentation can be determined in vivo. In these experiments it was about 10 kJ per 100 kJ digestible energy consumed.