Presently 3-methylhistidine excretion is widely used for monitoring the metabolic status of patients during different kinds of clinical conditions. Aim of the study was to reconsider its predicative value on the basis of a larger collective of healthy persons and to find a standardization independent from sex. Therefore endogenous 3-methylhistidine release of 40 healthy adults (24 women and 16 men) was measured and related to body weight, body surface area, arm muscle circumference, and nitrogen and creatinine excretion. A positive correlation could be observed only for 3-methylhistidine and creatinine excretion and that to the same extent both for females and males. Assuming that the excreted 3-methylhistidine is mainly originating from muscle protein the calculated daily protein breakdown amounted for women 39.9 g and 68.1 g for men. No difference between females and males could be observed in the percentual turnover of myofibrillar protein which has been estimated with 0.90 and 0.98 respectively. We interpret our results saying that endogenous 3-methylhistidine excretion is a valuable indicator for muscle protein breakdown in humans with intact kidney function. For the assessment of muscle proteolysis by 3-methylhistidine excretion in heterogenous groups of patients it is recommended to use the 3-methylhistidine/creatinine ratio or the percental turnover of myofibrillar protein.