The nutritional value of N-acetyl-L-tryptophan (NALT) given intravenously was examined in unrestrained adult rats. They were given solutions of 4% Vuj pattern amino acids, 24.5% glucose, vitamins and electrolytes intravenously for 10 days. They were divided into three groups; one group was then injected intravenously with complete amino acid solution (T group), one with tryptophan-free amino acid solution (TF group) and one group with complete amino acid solution but with NALT instead of tryptophan (NALT). The biological value of the NALT group was 49.6; this value corresponded to 90% of that of the T group. Excretion of injected amino acids totalled about 2.0 to 3.0% in this experiment. The effects of NALT on carcass composition, certain blood constituents and tryptophan pyrrolase activity in the liver were also examined.