The mink is a strict carnivore and may therefore serve as a model for the cat. Current recommendations for protein supply for lactating mink are based on production experiments with preweaning kit growth as a measure of dietary adequacy (1,2). Recently, nitrogen balance and substrate oxidation have been used to obtain a more detailed knowledge of the protein utilization of the lactating mink (3-5). These results have shown that lactating mink dams are able to regulate protein oxidation rate and that milk yield, during the first 4 wk post-partum, was improved, and dam weight loss reduced, when protein supply was reduced below current recommendations and replaced with readily digestible carbohydrates (5,6). The effect of reduced protein supply on protein turnover is, however, still unknown. Tracer methodology with ¹5 N-labeled amino acids has been used to measure the whole-body protein turnover in humans (7), growing pigs (8), and growing rats (9). In adult cats, both protein synthesis and breakdown were lower when feeding a low- than when feeding a high-protein diet [20 vs. 70% of metabolizable energy (ME)5 from protein] (10). The objectives of this study were therefore to develop a ¹5N-glycine endproduct method for measurements of protein turnover in lactating mink dams and to evaluate if it was affected by different levels of dietary protein supply.