A 4 X 4 Latin square design with an extra period was used to determine the influence of different levels of valine (Val) on the nitrogen metabolism of sheep receiving all nutrients by infusion. Glucose, amino acids and water-soluble vitamins were introduced in the jugular vein while inorganic nutrients and volatile fatty acids were given intraruminally. The amino acid mixture contained 1, 3, 5 or 7 g of Val with varying amounts of glycine (Gly) to balance the nitrogen level. The infusion in the extra period was isonitrogenous but contained no Val. When Val was omitted from the infusion, nitrogen balance (NB) was significantly reduced and plasma urea nitrogen (PUN) was significantly increased compared with all levels of added Val. The level of Val infusion that supported the numerically highest NB and the lowest PUN and urinary urea nitrogen (UUN) was 3 g/day, although NB and PUN, at all levels of added Val, were not statistically different. Plasma free Val increased abruptly at infusion levels above 3 g of infusion. It was concluded that Val is metabolically essential for the sheep and the best estimate of the requirements would be 200 mg/(kg body weight)0.75 per day in animals of this age and sex.