Whereas skin protein synthesis can be measured with different approaches, no method potentially applicable in humans is available for measurement of skin protein breakdown. To that end, we measured mixed skin fractional protein breakdown (FBR) in a rat model by use of a stable isotope method (tracee release method) originally developed to measure muscle protein breakdown. Skin mixed protein and collagen fractional synthesis rates (FSR) were also measured. A primed continuous infusion of L-[ring-²H₅]phenylalanine and -[5,5,5-²H₃]ketoisocaproate (KIC) was given for 6 h. Arterial and skin phenylalanine and leucine free enrichments were measured at plateau (5-6 h) and during the decay that followed after the infusion was stopped. Skin FBR (%/h) was 0.260 ± 0.011 with phenylalanine and 0.201 ± 0.032 with KIC/leucine [P = not significant (NS)]. Mixed skin FSR (%/h) was 0.169 ± 0.055 with phenylalanine and 0.146 ± 0.020 with KIC/leucine (P = NS). Collagen FSR was 0.124 ± 0.023%/h (P = NS vs. mixed protein FSR). The tracee release method is a sensitive method for measurement of skin protein breakdown; however, given the high intersubject variability of FSR, the calculation of skin net balance is not advisable.