The experiments reported in this series were designed to investigate the metabolism of lysine, leucine and methionine in lambs fed roughage diets supplemented with various levels of protein. and to compare these amino acids as tracers of whole-body protein metabolism in ruminants. Amino acid kinetics were estimated using a 12-h intravenous infusion of L-[U14C]-lysine. L-[35S]-methionine and L-[4,5-3H(n)] -leucine. The kinetics of methionine and a comparison of estimates of whole-body protein turnover are discussed in a following article (Cronje et al., 1992). The flux, oxidation and incorporation of lysine into protein were increased by protein supplementation (P < 0.05). The proportion of flux oxidized was increased (P = 0.08) by protein supplementation, and the fraction incorporated into protein was decreased (P = 0.07). It is suggested that the supply of lysine was limiting for protein synthesis when the low protein diet was fed. Leucine flux rate was increased by protein supplementation (P < 0.01). There was no change in fractional oxidation rate (23% of flux rate) with increasing’ protein supply (P > 0.05), but the total amount of leucine oxidized was increased by protein supplementation (P < 0.01). Similarly, the amount of leucine incorporated into protein was increased (P < 0.01) but not the proportion of leucine flux (P > 0.05). It was concluded that the supply of leucine available from the low-protein diet was lower than the requirement, and that while this restriction was alleviated by further protein supplementation, the efficiency of protein synthesis was still sub-optimal, possibly as a result of an insufficient supply of methionine. It is suggested that considerable scope exists for increasing the efficiency of utilization of existing protein supplements by lambs fed roughage diets via supplementation with specific amino acids protected from rumen degradation.