Cattle fed forage based diets often absorb a high proportion of dietary N as ammonia (NH3N) and this must be detoxified in the liver to urea. It has been suggested that this process utilises both energy and amino acids and may contribute to the lower efficiency of utilisation of dietary nitrogen and energy observed in forage fed ruminants (Fitch et al, 1989). However it is not known to what extent ammonia conversion to urea requires simultaneous deamination of amino acids. We report an experiment which examined hepatic nitrogen and energy metabolism at two different rates of NH3 arrival at the liver, in response an infusion of arginine to increase urea cycle flux. Arginine stimulates the hepatic synthesis of N-acetyl glutamate which is an activator of carbamoyl-phosphate synthase, the rate limiting step of the urea cycle (Meijer et al, 1990).