Muscle protein catabolism has been evaluated using the excretion of urinary 3-methylhistidine (3-MEH) is six normal male and six normal female subjects and in four surgical patients, two of whom developed febrile episodes during the course of their study. In addition, their nutritional status was also evaluated using percentage body weight losses before hospital admittance, creatinine-height ratios, and, in two patients, serum alkaline ribonuclease levels. The results indicate that: 1) prolonged starvation may produced decreased 3-MEH excretion because of an adaptive diminution of muscle breakdown in sustained starvation, decreased 3-MEH excretion also may simply reflect diminished lean body mass, 3-MEH excretion may be increased above basal levels because of superimposed stresses such as fever, and the acute phases of starvation produce increased levels of 3-MEH excretion until adaptive mechanisms occur; 2) creatinine-height ratios are low in starvation, and increase not only with improved nutrition but in response to fever and stress of operation, even when these are superimposed on malnutrition; and 3) alkaline RNAase levels are elevated in malnutrition and decrease with improved nutrition but in response to fever and stress of operation, even when these are superimposed on malnutrition; and 3) alkaline RNAase levels are elevated in malnutrition and decrease with improved nutrition. The enzyme may also be elevated by the stress of operations.