In 39 healthy adult volunteers and nine patients receiving total parenteral nutrition (TPN), determinations were made of the urinary excretion of 3-methylhistidine (3-Mehis) to estimate the degree to which the urinary output of 3-Mehis reflects the status of protein nutrition. In 19 males and 10 females of the former group, measurements were made of the amount of 3-Mehis and of creatinine (Cr) in 24-hr urine samples. The urinary output of 3-Mehis was also measured in two subgroups of five healthy adult male subjects who were fed one of the two different diets prescribed: meat (M) and meat-free (F) diet. Furthermore, in one of these subgroups which was fed on F diet, the urinary output of 3-Mehis was measured both while the subject was kept at bed rest and while the subject was allowed to live a normal life. From these measurements the standard value was determined for the urine 3-Mehis level and the 3-Mehis/Cr ratio in 24-hr urine in healthy adult human subjects of both sexes. In nine patients maintained on TPN these indices of nutritional status were measured before and at 4 weeks of TPN. The time course of the indices was also investigated. The results obtained suggest that both the urinary output of 3-Mehis and the 3-Mehis/Cr ratio can provide a sensitive index of the status of protein nutrition.