The urinary excretion of sulfur-containing compounds was studied before, on the third, and on the seventh day of fasting in 10 healthy subjects. The excretion of total sulfur, inorganic sulfate, ester sulfate, "non-sulfate sulfur", methionine, cystathionine, cysteine, N-acetylcysteine, taurine, thiosulfate and thiocyanate was decreased during fasting, whereas the excretion of mercaptoacetate was unaltered and that of mercaptolactate increased. The excretion of inorganic sulfate, taurine and thiocyanate was also decreased when calculated relative to that of total sulfur, suggesting that these compounds are derived mainly from dietary sulfur amino acids. The output of ester sulfate, methionine, cystathionine, cysteine and thiosulfate was unaltered in relation to that of total sulfur, indicating that these compounds are derived from both dietary and endogenous sulfur amino acids, liberated during protein catabolism. By contrast, the excretion of mercaptolactate and mercaptoacetate was increased relative to that of total sulfur, suggesting that these compounds are derived mainly from endogenous sulfur amino acids formed by the enhanced protein catabolism seen during fasting.