Urinary 3-methylhistidine (3MH) excretion has been proposed as a noninvasive in vivo marker of muscle protein breakdown, but such analysis requires quantitative collection of urine and yields few details about the metabolism of 3MH. In this study, we propose that data from a single bolus dose of tracer and serial blood samples over 72 h can be described by a kinetic model that defines 3MH metabolism in humans. Plasma concentration of the tracer was described by a linear time-invariant three-compartment model. The model defines masses and fluxes of 3MH within the subjects and, in particular, the intracellular de novo production of 3MH. The de novo production of 3MH as calculated by the model was not different from that calculated via the traditional collection of urinary 3MH (3.09 vs 2.57 mumol.kg-1.day-1, respectively; P > 0.30). These data indicate that 3MH production can be measured by a compartmental model that can be used to measure muscle proteolysis without quantitative urine collections.